Artist's concept of the hypothetical "Planet Nine." Could it have moons? Credit: NASA/JPL-Caltech/Robert Hurt
Ever since its existence was first proposed, the evidence for Planet 9 continues to mount. But of course, said evidence has been entirely indirect, consisting mostly of studies that show how the orbits of Trans-Neptunian Objects (TNOs) are consistent with a large object crossing their path. However, evidence is also emerging that comes from the center of the Solar System itself.
This latest line of evidence comes from Caltech, where researchers Elizabeth Bailey, Konstantin Batygin, and Michael E. Brown (the latter of whom were the ones who first proposed Planet 9’s existence) have published a new study linking solar obliquity to the existence of Planet 9. Essentially, they claim that the axial tilt of the Sun (6°) could be due to the gravitational influence a large planet with an extreme orbit.
To recap, the issue of Planet was first raised in 2014 by astronomers Scott Sheppard and Chadwick Trujillo. Noting the similarities in the orbits of distant Trans-Neptunian Objects (TNOs), they postulated that a massive object was likely influencing them. This was followed in 2016 by Konstantin Batygin and Michael E. Brown of Caltech suggesting that an undiscovered planet was the culprit.
The orbits of several KBOs provide indications about the possible existence of Planet 9. Credit: Caltech/R. Hurt (IPAC)
Calling this body Planet 9, they speculated that it had a mass 10 times greater than that of Earth, and took 20,000 years to complete a single orbit of our Sun. They also speculated that its orbit was tilted relative to the other planets of our Solar System, and extremely eccentric. And little by little, examinations of other Solar bodies have shown that Planet 9 is likely out there.
For the sake of their study – “Solar Obliquity Induced by Planet Nine“, which was recently published in the Astrophysical Journal – the research team (led by Bailey) looked to the obliquity of the Sun. As they state in their paper, the six-degree axial tilt of the Sun can only be explained in one of two ways – either as a result of an asymmetry that was present during the formation of Solar System, or because of an external source of gravity.
To test this hypothesis, Bailey, Batygin and Brown used an analytic model to test how interactions between Planet 9 and the rest of the Solar System would effect their orbits over the course of the last 4.5 billion years. As Elizabeth Bailey, a graduate student at Caltech’s Division of Geological and Planetary Sciences and the lead author on the paper, told Universe Today via email:
“We simulated the solar system’s motion. Planet 9 forces the solar system to slowly wobble. If Planet 9 is out there, we are in the process of wobbling right now, as we speak! But it happens very slowly, a few degrees tilt per billion years. Meanwhile the sun is not wobbling much, so it looks like the sun is tilted. A range of Planet 9 parameters cause exactly the configuration of the sun that we see today.
Animated diagram showing the spacing of the Solar Systems planet’s, the unusually closely spaced orbits of six of the most distant KBOs, and the possible “Planet 9”. Credit: Caltech/nagualdesign
In the end, they concluded that the Sun’s obliquity could only be explained by the influence of giant planet with an extreme orbit, one that is consistent with the characteristics attributed to Planet 9. In other words, the existence of Planet 9 offers an explanation for the Sun’s peculiar behavior, something which has remained a mystery until now.
“Planet Nine was first hypothesized because the orbits of objects in the outer reaches of the solar system are confined in physical space,” said Bailey. “Those orbits would be all over the place unless something is currently stopping them. The only explanation so far is Planet Nine. For over 150 years, people have wondered why the sun is tilted. Personally I’d say that Planet 9 offers the first satisfying explanation. If it exists, it tilted the sun.”
The Arizona research team was led by Renu Malhotra, a Regents’ Professor of Planetary Sciences in the University of Arizona’s Lunar and Planetary Lab. For the sake of their study, titled “Corralling a Distant Planet with Extreme Resonant Kuiper Belt Objects“, they examined the orbital patterns of four extreme Kuiper Belt Objects (KBOs), which have the longest orbital periods of any known objects.
Artist’s impression of the the possible Planet 9 at the edge of the Solar System. Credit: Robin Dienel/Carnegie Science
According to their calculations, the presence of a massive planet – one that would complete an orbit around the Sun every 17,117 years, and at an average distance (semimajor axis) of 665 AU – would explain the orbital pattern of these four objects. These results were consistent with the estimates concerning the orbital period of Planet 9, its orbital path, and it mass.
“We analyzed the data of these most distant Kuiper Belt objects,” Malhotra said, “and noticed something peculiar, suggesting they were in some kind of resonances with an unseen planet… Our paper provides more specific estimates for the mass and orbit that this planet would have, and, more importantly, constraints on its current position within its orbit.”
Looks like Planet 9’s days of hiding in the outer Solar System may be numbered!
Special Guests:
Mike Brown is the Richard and Barbara Rosenberg Professor of Planetary Astronomy at CalTech. Konstantin Batygin is Assistant Professor of Planetary Science at CalTech. They’ll be here discussing their discovery of Planet 9 and what’s been happening since that amazing announcement.
We’ve had an abundance of news stories for the past few months, and not enough time to get to them all. So we’ve started a new system. Instead of adding all of the stories to the spreadsheet each week, we are now using a tool called Trello to submit and vote on stories we would like to see covered each week, and then Fraser will be selecting the stories from there. Here is the link to the Trello WSH page (http://bit.ly/WSHVote), which you can see without logging in. If you’d like to vote, just create a login and help us decide what to cover!
We record the Weekly Space Hangout every Friday at 12:00 pm Pacific / 3:00 pm Eastern. You can watch us live on Universe Today or the Universe Today YouTube page.
Artist's impression of Planet Nine, blocking out the Milky Way. The Sun is in the distance, with the orbit of Neptune shown as a ring. Credit: ESO/Tomruen/nagualdesign
On January 20th, 2016, researchers Konstantin Batygin and Michael E. Brown of Caltech announced that they had found evidence that hinted at the existence of a massive planet at the edge of the Solar System. Based on mathematical modeling and computer simulations, they predicted that this planet would be a super-Earth, two to four times Earth’s size and 10 times as massive. They also estimated that, given its distance and highly elliptical orbit, it would take 10,000 – 20,000 years to orbit the Sun.
Since that time, many researchers have responded with their own studies about the possible existence of this mysterious “Planet 9”. One of the latest comes from the University of Arizona, where a research team from the Lunar and Planetary Laboratory have indicated that the extreme eccentricity of distant Kuiper Belt Objects (KBOs) might indicate that they crossed paths with a massive planet in the past.
For some time now, it has been understood that there are a few known KBOs who’s dynamics are different than those of other belt objects. Whereas most are significantly controlled by the gravity of the gas giants planets in their current orbits (particularly Neptune), certain members of the scattered disk population of the Kuiper Belt have unusually closely-spaced orbits.
The orbits of Neptune (magenta), Sedna (dark magenta), a series of Kuiper belt objects (cyan), and the hypothetical Planet 9 (orange). Credit: Caltech/R. Hurt (IPAC)
When Batygin and Brown first announced their findings back in January, they indicated that these objects instead appeared to be highly clustered with respect to their perihelion positions and orbital planes. What’s more, their calculation showed that the odds of this being a chance occurrence were extremely low (they calculated a probability of 0.007%).
Instead, they theorized that it was a distant eccentric planet that was responsible for maintaining the orbits of these KBOs. In order to do this, the planet in question would have to be over ten times as massive as Earth, and have an orbit that lay roughly on the same plane (but with a perihelion oriented 180° away from those of the KBOs).
Such a planet not only offered an explanation for the presence of high-perihelion Sedna-like objects – i.e. planetoids that have extremely eccentric orbits around the Sun. It would also help to explain where distant and highly inclined objects in the outer Solar System come from, since their origins have been unclear up until this point.
In a paper titled “Coralling a distant planet with extreme resonant Kuiper belt objects“, the University of Arizona research team – which included Professor Renu Malhotra, Dr. Kathryn Volk, and Xianyu Wang – looked at things from another angle. If in fact Planet 9 were crossing paths with certain high-eccentricity KBOs, they reasoned, it was a good bet that its orbit was in resonance with these objects.
Pluto and its cohorts in the icy-asteroid-rich Kuiper Belt beyond the orbit of Neptune. Credit: NASA
To break it down, small bodies are ejected from the Solar System all the time due to encounters with larger objects that perturb their orbits. In order to avoid being ejected, smaller bodies need to be protected by orbital resonances. While the smaller and larger objects may pass within each others’ orbital path, they are never close enough that they would able to exert a significant influence on each other.
This is how Pluto has remained a part of the Solar System, despite having an eccentric orbit that periodically cross Neptune’s path. Though Neptune and Pluto cross each others orbit, they are never close enough to each other that Neptune’s influence would force Pluto out of our Solar System. Using this same reasoning, they hypothesized that the KBOs examined by Batygin and Brown might be in an orbital resonance with the Planet 9.
As Dr. Malhotra, Volk and Wang told Universe Today via email:
“The extreme Kuiper belt objects we investigate in our paper are distinct from the others because they all have very distant, very elliptical orbits, but their closest approach to the Sun isn’t really close enough for them to meaningfully interact with Neptune. So we have these six observed objects whose orbits are currently fairly unaffected by the known planets in our Solar System. But if there’s another, as yet unobserved planet located a few hundred AU from the Sun, these six objects would be affected by that planet.”
After examining the orbital periods of these six KBOs – Sedna, 2010 GB174, 2004 VN112, 2012 VP113, and 2013 GP136 – they concluded that a hypothetical planet with an orbital period of about 17,117 years (or a semimajor axis of about 665 AU), would have the necessary period ratios with these four objects. This would fall within the parameters estimated by Batygin and Brown for the planet’s orbital period (10,000 – 20,000 years).
Animated diagram showing the spacing of the Solar Systems planet’s, the unusually closely spaced orbits of six of the most distant KBOs, and the possible “Planet 9”. Credit: Caltech/nagualdesign
Their analysis also offered suggestions as to what kind of resonance the planet has with the KBOs in question. Whereas Sedna’s orbital period would have a 3:2 resonance with the planet, 2010 GB174 would be in a 5:2 resonance, 2994 VN112 in a 3:1, 2004 VP113 in 4:1, and 2013 GP136 in 9:1. These sort of resonances are simply not likely without the presence of a larger planet.
“For a resonance to be dynamically meaningful in the outer Solar System, you need one of the objects to have enough mass to have a reasonably strong gravitational effect on the other,” said the research team. “The extreme Kuiper belt objects aren’t really massive enough to be in resonances with each other, but the fact that their orbital periods fall along simple ratios might mean that they each are in resonance with a massive, unseen object.”
But what is perhaps most exciting is that their findings could help to narrow the range of Planet 9’s possible location. Since each orbital resonance provides a geometric relationship between the bodies involved, the resonant configurations of these KBOs can help point astronomers to the right spot in our Solar System to find it.
But of course, Malhotra and her colleagues freely admit that several unknowns remain, and further observation and study is necessary before Planet 9 can be confirmed:
“There are a lot of uncertainties here. The orbits of these extreme Kuiper belt objects are not very well known because they move very slowly on the sky and we’ve only observed very small portions of their orbital motion. So their orbital periods might differ from the current estimates, which could make some of them not resonant with the hypothetical planet. It could also just be chance that the orbital periods of the objects are related; we haven’t observed very many of these types of objects, so we have a limited set of data to work with.”
Estimates of Planet Nine’s “possible” and “probable” zones. by French scientists based on a careful study of Saturn’s orbit and using mathematical models. Source: CNRS, Cote d’Azur and Paris observatories. Credit: Bob King
Ultimately, astronomers and the rest of us will simply have to wait on further observations and calculations. But in the meantime, I think we can all agree that the possibility of a 9th Planet is certainly an intriguing one! For those who grew up thinking that the Solar System had nine planets, these past few years (where Pluto was demoted and that number fell to eight) have been hard to swallow.
But with the possible confirmation of this Super-Earth at the outer edge of the Solar System, that number could be pushed back up to nine soon enough!
An artist's conception of 2007 OR10, nicknamed Snow White. Astronomers suspect that its rosy color is due to the presence of irradiated methane. [Credit: NASA]
Over the course of the past decade, more and more objects have been discovered within the Trans-Neptunian region. With every new find, we have learned more about the history of our Solar System and the mysteries it holds. At the same time, these finds have forced astronomers to reexamine astronomical conventions that have been in place for decades.
Consider 2007 OR10, a Trans-Neptunian Object (TNO) located within the scattered disc that at one time went by the nicknames of “the seventh dwarf” and “Snow White”. Approximately the same size as Haumea, it is believed to be a dwarf planet, and is currently the largest object in the Solar System that does not have a name.
Discovery and Naming:
2007 OR10 was discovered in 2007 by Meg Schwamb, a PhD candidate at Caltech and a graduate student of Michael Brown, while working out of the Palomar Observatory. The object was colloquially referred to as the “seventh dwarf” (from Snow White and the Seven Dwarfs) since it was the seventh object to be discovered by Brown’s team (after Quaoar in 2002, Sedna in 2003, Haumea and Orcus in 2004, and Makemake and Eris in 2005).
Comparison of Sedna with the other largest TNOs and with Earth (all to scale). Credit: NASA/Lexicon
At the time of its discovery, the object appeared to be very large and very white, which led to Brown giving it the other nickname of “Snow White”. However, subsequent observation has revealed that the planet is actually one of the reddest in the Kuiper Belt, comparable only to Haumea. As a result, the nickname was dropped and the object is still designated as 2007 OR10.
The discovery of 2007 OR10 would not be formally announced until January 7th, 2009.
Size, Mass and Orbit:
A study published in 2011 by Brown – in collaboration with A.J. Burgasser (University of California San Diego) and W.C. Fraser (MIT) – 2007 OR10’s diameter was estimated to be between 1000-1500 km. These estimates were based on photometry data obtained in 2010 using the Magellan Baade Telescope at the Las Campanas Observatory in Chile, and from spectral data obtained by the Hubble Space Telescope.
However, a survey conducted in 2012 by Pablo Santos Sanz et al. of the Trans-Neptunian region produced an estimate of 1280±210 km based on the object’s size, albedo, and thermal properties. Combined with its absolute magnitude and albedo, 2007 OR10 is the largest unnamed object and the fifth brightest TNO in the Solar System. No estimates of its mass have been made as of yet.
2007 OR10 also has a highly eccentric orbit (0.5058) with an inclination of 30.9376°. What this means is that at perihelion, it is roughly 33 AU (4.9 x 109 km/30.67 x 109 mi) from our Sun while at aphelion, it is as distant as 100.66 AU (1.5 x 1010 km/9.36 x 1010 mi). It also has an orbital period of 546.6 years, which means that the last time it was at perihelion was 1857 and it won’t reach aphelion until 2130. As such, it is currently the second-farthest known large body in the Solar System, and will be farther out than both Sedna and Eris by 2045.
Composition:
According to the spectral data obtained by Brown, Burgasser and Fraser, 2007 OR10 shows infrared signatures for both water ice and methane, which indicates that it is likely similar in composition to Quaoar. Concurrent with this, the reddish appearance of 2007 OR10 is believed to be due to presence of tholins in the surface ice, which are caused by the irradiation of methane by ultraviolet radiation.
The presence of red methane frost on the surfaces of both 2007 OR10 and Quaoar is also seen as an indication of the possible existence of a tenuous methane atmosphere, which would slowly evaporate into space when the objects are closer to the Sun. Although 2007 OR10 comes closer to the Sun than Quaoar, and is thus warm enough that a methane atmosphere should evaporate, its larger mass makes retention of an atmosphere just possible.
Also, the presence of water ice on the surface is believed to imply that the object underwent a brief period of cryovolcanism in its distant past. According to Brown, this period would have been responsible not only for water ice freezing on the surface, but for the creation of an atmosphere that included nitrogen and carbon monoxide. These would have been depleted rather quickly, and a tenuous atmosphere of methane would be all that remains today.
However, more data is required before astronomers can say for sure whether or not 2007 OR10 has an atmosphere, a history of cryovolcanism, and what its interior looks like. Like other KBOs, it is possible that it is differentiated between a mantle of ices and a rocky core. Assuming that there is sufficient antifreeze, or due to the decay of radioactive elements, there may even be a liquid-water ocean at the core-mantle boundary.
Classification:
Though it is too difficult to resolve 2007 OR10’s size based on direct observation, based on calculations of 2007 OR10’s albedo and absolute magnitude, many astronomers believe it to be of sufficient size to have achieved hydrostatic equilibrium. As Brown stated in 2011, 2007 OR10 “must be a dwarf planet even if predominantly rocky”, which is based on a minimum possible diameter of 552 km and what is believed to be the conditions under which hydrostatic equilibrium occurs in cold icy-rock bodies.
That same year, Scott S. Sheppard and his team (which included Chad Trujillo) conducted a survey of bright KBOs (including 2007 OR10) using the Palomar Observatory’s 48 inch Schmidt telescope. According to their findings, they determined that “[a]ssuming moderate albedos, several of the new discoveries from this survey could be in hydrostatic equilibrium and thus could be considered dwarf planets.”
Currently, nothing is known of 2007 OR10’s mass, which is a major factor when determining if a body has achieved hydrostatic equilibrium. This is due in part to there being no known satellite(s) in orbit of the object, which in turn is a major factor in determining the mass of a system. Meanwhile, the IAU has not addressed the possibility of accepting additional dwarf planets since before the discovery of 2007 OR10 was announced.
Alas, much remains to be learned about 2007 OR10. Much like it’s Trans-Neptunian neighbors and fellow KBOs, a lot will depend on future missions and observations being able to learn more about its size, mass, composition, and whether or not it has any satellites. However, given its extreme distance and fact that it is currently moving further and further away, opportunities to observe and explore it via flybys will be limited.
However, if all goes well, this potential dwarf planet could be joining the ranks of such bodies as Pluto, Eris, Ceres, Haumea and Makemake in the not-too-distant future. And with luck, it will be given a name that actually sticks!
We’ve only had blurry images of Pluto up until New Horizons. So what did we learn when we got up close and personal with Pluto and its moons?
Clyde Tombaugh first discovered Pluto in 1930. He saw only see a single speck of light moving slowly in front of the background stars as he flipped photographic plates back and forth. Sadly, this was the best anyone could do for decades. Even the mighty Hubble, the most sensitive instrument ever focused on Pluto, could only resolve a few grainy pixels.
It’s because Pluto is really really far away: 7.5 billion kilometers. Just the light alone from there takes over 4 hours to reach us. In order to get any more information, humanity needed to reach out and send a spacecraft to Pluto, and photograph it, up close and personal.
In 1989, Alan Stern and a group of planetary scientists began working on a mission. Their work culminated in NASA’s New Horizons spacecraft, launched in 2006, beginning a 9 and a half year journey. And unless you’ve been living in a lunar lava tube, you know that New Horizons finally reached its destination in mid July 2015, passing a narrow 12,472 kilometers above the surface.
For the very first time in human history, we saw a member of the Kuiper Belt right up in it’s business. And now I retire these old low quality images Pluto! Begone artist’s illustrations!
From here on out, we’re all about sick high def photos of the surface and its moons. I for one am going to revel in them for a while.
So fashion shoots aside, what did we actually learn about Pluto? The primary mission was to map the geography of Pluto and its biggest moon, Charon. It would study the surface chemistry of these icy worlds, and measure their atmospheres, if they even exist at all.
The mission had a few other objectives, and of course, planetary scientists knew that the spacecraft would just surprise us with stuff we never expected. Kuiper Belt objects like Pluto and Charon are ancient; geologists expected them to be pockmarked with craters, large and small.
Views of Pluto during New Horizons’ approach. Credit: NASA/Damian Peach
Surprisingly, New Horizons showed relatively smooth surfaces on both worlds. Pluto has a Texas-sized region newly named Sputnik Planum, where exotic ices flow like glaciers. Frozen nitrogen, carbon dioxide and methane ices act just like the ones we have here on Earth. We can see from the relative lack of craters that this process is still happening.
Pluto has mountains. Mountains! Close ups show a young range with peaks as high as 11,000 feet, or 3,500 meters. Here’s the crazy part. Those exotic chemicals that act like snow and ice? They’re not hard enough to make mountain peaks like this.
At extreme cold temperatures, water ice becomes as hard as rock. These mountains are made of ice, and they’re very young, probably less than 100 million years old. There could be plate tectonics on Pluto, but with ice, not rock. My mind is blown.
Pluto’s moon Charon has a huge chasm longer and deeper than the Grand Canyon in Arizona and although scientists hoped to see an atmosphere, the reality was beyond anyone’s expectations.
Backlit by the sun, Pluto’s atmosphere rings its silhouette like a luminous halo in this image taken by NASA’s New Horizons spacecraft around midnight EDT on July 15. This global portrait of the atmosphere was captured when the spacecraft was about 1.25 million miles (2 million kilometers) from Pluto and shows structures as small as 12 miles across. The image, delivered to Earth on July 23, is displayed with north at the top of the frame. Credits: NASA/JHUAPL/SwRI
New Horizons detected a thin nitrogen atmosphere at Pluto. It could be snowing nitrogen on Pluto right now. There could be faint winds, since there are regions on Pluto that look like they might have undergone weathering.
Take a look at this photograph as New Horizons zipped away. You can see the atmosphere clearly surrounding the dwarf planet, interacting with the solar wind and creating a tail that stretches away from the Sun.
Here’s my favorite thing we learned. Pluto is about 80 km larger than previous estimates, which makes it the largest Kuiper Belt Object found so far. Even bigger than Eris, which is still a little more massive. So maybe it’s time to revisit that Pluto planethood debate again. I’m just messing with you. No good will ever come from having that debate. It will only end in tears.
Interestingly, the data connection between Earth and New Horizons is tenuous. Possibly the worst internet since AOL. It can only transmit back about 1kb of data per second, which means that we’ll need to wait about 16 months for the photographs and data to be sent home during the first few days of the flyby.
As an extra bonus, this isn’t the last we’re going to hear from New Horizons. Because it’s so far away, as the spacecraft can only trickle data back to Earth. It’s going to take almost 2 years for all the images and measurements it gathered during its flyby to get back to Earth for scientists to study. Expect many more discoveries and announcements over the coming years, and more videos from us.
Now that Pluto has finally been explored, where do you think we should go next in the Solar System? Tell us in the comments below.
An artist’s conception shows the New Horizons spacecraft flying past a Pluto-like object in the Kuiper Belt, the ring of icy material that lies billions of miles away from the sun. (Credit: Alex Parker / NASA / JHUAPL / SwRI)
NASA and the science team behind the New Horizons mission to Pluto and beyond have settled on the popular choice for the spacecraft’s next flyby: It’s 2014 MU69, an icy object a billion miles beyond Pluto that’s thought to be less than 30 miles (45 kilometers) wide.
That’s 10 times bigger than, say, the comet targeted by the European Space Agency’s Rosetta probe – but on the order of 1 percent as wide as Pluto. The New Horizons team suspects that 2014 MU69 represents a primordial object in the Kuiper Belt, the vast ring of icy material that lies beyond the orbit of Neptune.
Studying such a Kuiper Belt object, or KBO, should satisfy the mission’s post-Pluto objective of documenting the diversity of worlds at the solar system’s edge. “It is just the kind of ancient KBO, formed where it orbits now, that the Decadal Survey desired us to fly by,” New Horizons principal investigator Alan Stern said Friday in a NASA news release.
This chart shows the path of NASA’s New Horizons spacecraft toward its next potential target, the Kuiper Belt object 2014 MU69, also known as PT1. Other dwarf planets are indicated on the chart as well. (Credit: Alex Parker / NASA / JHUAPL / SwRI)
2014 MU69, also known as Potential Target 1 or PT1, was one of three objects identified after a months-long search that drew upon the observing power of the Hubble Space Telescope. Although an alternate target known as PT3 was somewhat brighter and probably bigger, PT1 was favored because there’s a 100 percent chance of reaching it with the fuel that was left on the New Horizons spacecraft after last month’s big Pluto flyby.
The New Horizons team has planned a series of four maneuvers in October and November to send the piano-sized probe toward 2014 MU69, but NASA won’t be able to give the final go-ahead for the extended mission until the team makes a formal proposal in 2016. If NASA gives the green light, the flyby is due to take place on Jan. 1, 2019.
In the meantime, New Horizons is continuing to send back imagery and other data that have been stored up since it flew past Pluto on July 14. New pictures should be released starting in a week or so.
Discovered in 2005, Makemake, a Kuiper Belt Object (KBO) has . Credit: NASA
In 2003, astronomer Mike Brown and his team from Caltech began a discovery process which would change the way we think of our Solar System. Initially, it was the discovery of a body with a comparable mass to Pluto (Eris) that challenged the definition of the word “planet”. But in the months and years that followed, more discoveries would be made that further underlined the need for a new system of classification.
This included the discovery of Haumea, Orcus and Salacia in 2004, and Makemake in 2005. Like many other Trans-Neptunian Objects (TNOs) and Kuiper Belt Objects (KBOs) discovered in the past decade, this planet’s status is the subject of some debate. However, the IAU was quick to designate it as the fourth dwarf planet in our Solar System, and the third “Plutoid“.
Discovery and Naming:
Makemake was discovered on March 31st, 2005, at the Palomar Observatory by a team consisting of Mike Brown, Chad Trujillo and David Rainowitz. The discovery was announced to the public on July 29th, 2005, coincident with the announcement of the discovery of Eris. Originally, Brown and his team had been intent on waiting for further confirmation, but chose to proceed after a different team in Spain announced the discovery of Haumea on July 27th.
The provisional designation of 2005 FY9 was given to Makemake when the discovery was first made public. Before that, the discovery team used the codename “Easterbunny” for the object, because it was observed shortly after Easter. In July of 2008, in accordance with IAU rules for classical Kuiper Belt Objects, 2005 FY9 was given the name of a creator deity.
Photograph of Makemake taken by the Hubble Space Telescope. Credit: NASA/Mike Brown
In order to preserve the object’s connection with Easter, the object was given a name derived from the mythos of the Rapa Nui (the native people of Easter Island) to whom Makemake is the creator God. It was officially classified as a dwarf planet and a plutoid by the International Astronomical Union (IAU) on July 19th, 2008.
Size, Mass and Orbit:
Based on infrared observations conducted by Brown and his team using the Spitzer Space Telescope, which were compared to similar observations made by the Herschel Space Telescope, an estimated diameter of 1,360 – 1,480 km was made. Subsequent observations made during the 2011 stellar occulation by Makemake produced estimated dimensions of 1502 ± 45 × 1430 ± 9 km.
Estimates of its mass place it in the vicinity of 4 x 10²¹ kg (4,000,000,000 trillion kg), which is the equivalent of 0.00067 Earths. This makes Makemake the third largest known Trans-Neptunian Object (TNOs) – smaller than Pluto and Eris, and slightly larger than Haumea.
Makemake has a slightly eccentric orbit (of 0.159), which ranges from 38.590 AU (5.76 billion km/3.58 billion mi) at perihelion to 52.840 AU ( 7.94 billion km or 4.934 billion miles) at aphelion. It has an orbital period of 309.09 Earth years, and takes about 7.77 Earth hours to complete a single sidereal rotation. This means that a single day on Makemake is less than 8 hours and a single year last as long as 112,897 days.
A selection of dwarf planets, sometimes considered trans-Neptunian objects depending on their interactions with the planet Neptune. Credit: NASA/STSci
As a classical Kuiper Belt Object, Makemake’s orbit lies far enough from Neptune to remain stable over the age of the Solar System. Unlike plutinos, which can cross Neptune’s orbit, classical KBOs are free from Neptune’s perturbation. Such objects have relatively low eccentricities (below 0.2) and orbit the Sun in much the same way the planets do. Makemake, however, is a member of the “dynamically hot” class of classical KBOs, meaning that it has a high inclination compared to others in its population.
Composition and Surface:
With an estimated mean density of 1.4–3.2 g/cm³, Makemake is believed to be differentiated between an icy surface and a rocky core. Like Pluto and Eris, the surface ice is believed to be composed largely of frozen methane (CH4) and ethane (C2H6). Though evidence exists for traces of nitrogen ice as well, it is nowhere near as prevalent as with Pluto or Triton.
Javier Licandro and his colleagues at the Instituto de Astrofisica de Canarias performed examinations of Makemake using the William Herschel Telescope and Telescopio Nazionale Galileo. According to their findings, Makemake has a very bright surface (with a surface albedo of 0.81) which means it closely resembles that of Pluto.
In essence, it appears reddish in color (significantly more so than Eris), which also indicates strong concentrations of tholins in the surface ice. This is consistent with the presence of methane ice, which would have turned red due to exposure to solar radiation over time.
Atmosphere:
During it’s 2011 occultation with an 18th-magnitutde star, Makemake abruptly blocked all of its light. These results showed that Makemake lacks a substantial atmosphere, which contradicted earlier assumptions about it having an atmosphere comparable to that of Pluto. However, the presence of methane and possibly nitrogen suggests that Makemake could have a transient atmosphere similar to that of Pluto when it reaches perihelion.
Artist’s impression of the surface of Makemake. Credit: NASA
Essentially, when Makemake is closest to the Sun, nitrogen and other ices would sublimate, forming a tenuous atmosphere composed of nitrogen gas and hydrocarbons. The existence of an atmosphere would also provide a natural explanation for the nitrogen depletion, which could have been lost over time through the process of atmospheric escape.
Moon:
In April of 2016, observations using the Hubble Space Telescope‘s Wide Field Camera 3 revealed that Makemake had a natural satellite – which was designated S/2015 (136472) 1 (nicknamed MK 2 by the discovery team). It is estimated to be 175 km (110 mi) km in diameter and has a semi-major axis at least 21,000 km (13,000 mi) from Makemake.
Exploration:
Currently, no missions have been planned to the Kuiper Belt for the purpose of conducting a survey of Makemake. However, it has been calculated that – based on a launch date of August 21st, 2024, and August 24th, 2036 – a flyby mission to Makemake could take just over 16 years, using a Jupiter gravity assist. On either occasion, Makemake would be approximately 52 AU from the Sun when the spacecraft arrives.
Makemake is now the fourth designated dwarf planet in the solar system, and the third Plutoid. In the coming years, it is likely to be joined several more objects in the Trans-Neptunian region that are similar in size, mass, and orbit. And assuming we mount a flyby to the region, we may discover many similar objects, and learn a great deal more about this one.
Tenth planet? Artists concept of the view from Eris with Dysnomia in the background, looking back towards the distant sun. Credit: Robert Hurt (IPAC)
Ask a person what Dysnomia refers to, and they might venture that it’s a medical condition. In truth, they would be correct. But in addition to being a condition that affects the memory (where people have a hard time remembering words and names), it is also the only known moon of the distant dwarf planet Eris.
In fact, the same team that discovered Eris a decade ago – a discovery that threw our entire notion of what constitutes a planet into question – also discovered a moon circling it shortly thereafter. As the only satellite that circles one of the most distant objects in our Solar System, much of what we know about this ball of ice is still subject to debate.
Discovery and Naming:
In January of 2005, astronomer Mike Brown and his team discovered Eris using the new laser guide star adaptive optics system at the W. M. Keck Observatory in Hawaii. By September, Brown and his team were conducting observations of the four brightest Kuiper Belt Objects – which at that point included Pluto, Makemake, Haumea, and Eris – and found indications of an object orbiting Eris.
Provisionally, this body was designated S/2005 1 (2003 UB³¹³). However, in keeping with the Xena nickname that his team was already using for Eris, Brown and his colleagues nicknamed the moon “Gabrielle” after Xena’s sidekick. Later, Brown selected the official name of Dysnomia for the moon, which seemed appropriate for a number of reasons.
For one, this name is derived from the daughter of the Greek god Eris – a daemon who represented the spirit of lawlessness – which was in keeping with the tradition of naming moons after lesser gods associated with the primary god. It also seemed appropriate since the “lawless” aspect called to mind actress Lucy Lawless, who portrayed Xena on television. However, it was not until the IAU’s resolution on what defined a planet – passed in August of 2006 – that the planet was officially designated as Dysnomia.
Size, Mass and Orbit:
The actual size of Dysnomia is subject to dispute, and estimates are based largely on the planet’s albedo relative to Eris. For example, the IAU and Johnston’s Asteroids with Satellites Database estimate that it is 4.43 magnitudes fainter than Eris and has an approximate diameter of between 350 and 490 km (217 – 304 miles)
However, Brown and his colleagues have stated that their observations indicate it to be 500 times fainter and between 100 and 250 km (62 – 155 miles) in diameter. Using the Herschel Space Observatory in 2012, Spanish astronomer Pablo Santo Sanz and his team determined that, provided Dysnomia has an albedo five times that of Eris, it is likely to be 685±50 km in diameter.
Eris and its moon, Dysnomia, as imaged by the W.M. Keck Observatory in Hawaii. Credits:NASA/ESA and M. Brown/Caltech
In 2007, Brown and his team also combined Keck and Hubble observations to determine the mass of Eris, and estimate the orbital parameters of the system. From their calculations, they determined that Dysnomia’s orbital period is approximately 15.77 days. These observations also indicated that Dysnomia has a circular orbit around Eris, with a radius of 37350±140 km. In addition to being a satellite of a dwarf planet, Dysnomia is also a Kuiper Belt Object (KBO) like Eris.
Composition and Origin:
Currently, there is no direct evidence to indicate what Dysnomia is made of. However, based on observations made of other Kuiper Belt Objects, it is widely believed that Dysnomia is composed primarily of ice. This is based largely on infrared observations made of Haumea (2003 EL61), the fourth largest object in the Kuiper Belt (after Eris, Pluto and Makemake) which appears to be made entirely of frozen water.
Astronomers now know that three of the four brightest KBOs – Pluto, Eris and Haumea – have one or more satellites. Meanwhile, of the fainter members, only about 10% are known to have satellites. This is believed to imply that collisions between large KBOs have been frequent in the past. Impacts between bodies of the order of 1000 km across would throw off large amounts of material that would coalesce into a moon.
Artist’s concept of Kuiper Belt Object Eris and its tiny satellite Dysnomia. The Hubble Space Telescope and Keck Observatory took images of Dysnomia’s movement from which astronomer Mike Brown (Caltech) precisely calculated Eris to be 27 percent more massive than Pluto. Credit: NASA/ESA/Adolph Schaller (for STScI)
This could mean that Dysnomia was the result of a collision between Eris and a large KBO. After the impact, the icy material and other trace elements that made up the object would have evaporated and been ejected into orbit around Eris, where it then re-accumulated to form Dysnomia. A similar mechanism is believed to have led to the formation of the Moon when Earth was struck by a giant impactor early in the history of the Solar System.
Since its discovery, Eris has lived up to its namesake by stirring things up. However, it has also helped astronomers to learn many things about this distant region of the Solar System. As already mentioned, astronomers have used Dysnomia to estimate the mass of Eris, which in turn helped them to compare it to Pluto.
While astronomers already knew that Eris was bigger than Pluto, but they did not know whether it was more massive. This they did by measuring the distance between Dysnomia and how long it takes to orbit Eris. Using this method, astronomers were able to discover that Eris is 27% more massive than Pluto is.
With this knowledge in hand, the IAU then realized that either Eris needed to be classified as a planet, or that the term “planet” itself needed to be refined. Ergo, one could make that case that it was the discovery of Dysnomia more than Eris that led to Pluto no longer being designated a planet.
Artist's impression of the New Horizons spacecraft in orbit around Pluto (Charon is seen in the background). Credit: NASA/JPL
Humans have been sending spacecraft to other planets, as well as asteroid and comets, for decades. But rarely have any of these ventured into the outer reaches of our Solar System. In fact, the last time a probe reached beyond the orbit of Saturn to explore the worlds of Neptune, Uranus, Pluto and beyond was with the Voyager 2 mission, which concluded back in 1989.
But with the New Horizons mission, humanity is once again peering into the outer Solar System and learning much about its planets, dwarf planets, planetoids, moons and assorted objects. And as of July 14th, 2015, it made its historic rendezvous with Pluto, a world that has continued to surprise and mystify astronomers since it was first discovered.
Background:
In 1980, after Voyager 1‘s flyby of Saturn, NASA scientists began to consider the possibility of using Saturn to slingshot the probe towards Pluto to conduct a flyby by 1986. This would not be the case, as NASA decided instead to conduct a flyby of Saturn’s moon of Titan – which they considered to be a more scientific objective – thus making a slingshot towards Pluto impossible.
Because no mission to Pluto was planned by any space agency at the time, it would be years before any missions to Pluto could be contemplated. However, after Voyager 2′s flyby of Neptune and Triton in 1989, scientists once again began contemplating a mission that would take a spacecraft to Pluto for the sake of studying the Kuiper Belt and Kuiper Belt Objects (KBOs).
Artist’s impression of the Voyager spacecraft in flight. Credit: NASA/JPL
In May 1989, a group of scientists, including Alan Stern and Fran Bagenal, formed an alliance called the “Pluto Underground”. Committed to the idea of mounting an exploratory mission to Pluto and the Kuiper Belt, this group began lobbying NASA and the US government to make it this plan a reality. Combined with pressure from the scientific community at large, NASA began looking into mission concepts by 1990.
During the course of the late 1990s, a number of Trans-Neptunian Objects (TNOs) were discovered, confirming the existence of the Kuiper Belt and spurring interest in a mission to the region. This led NASA to instruct the JPL to re-purpose the mission as a Pluto and KBO flyby. However, the mission was scrapped by 2000, owing to budget constraints.
Backlash over the cancellation led NASA’s Science Mission Directorate to create the New Frontiers program which began accepting mission proposals. Stamatios “Tom” Krimigis, head of the Applied Physics Laboratory’s (APL) space division, came together with Alan Stern to form the New Horizons team. Their proposal was selected from a number of submissions, and officially selected for funding by the New Frontiers program in Nov. 2001.
Despite additional squabbles over funding with the Bush administration, renewed pressure from the scientific community allowed the New Horizons team managed to secure their funding by the summer of 2002. With a commitment of $650 million for the next fourteen years, Stern’s team was finally able to start building the spacecraft and its instruments.
Engineers working on the New Horizons spacecraft’s Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument. Credit: NASA
Mission Profile:
New Horizons was planned as a voyage to the only unexplored planet in the Solar System, and was originally slated for launch in January 2006 and arrival at Pluto in 2015. Alan Stern was selected as the mission’s principal investigator, and construction of the spacecraft was handled primarily by the Southwest Research Institute (SwRI) and the Johns Hopkins Applied Physics Laboratory, with various contractor facilities involved in the navigation of the spacecraft.
Meanwhile, the US Naval Observatory (USNO) Flagstaff Station – in conjunction with NASA and JPL – was responsible for performing navigational position data and related celestial frames. Coincidentally, the UNSO Flagstaff station was where the photographic plates that led to the discovery of Pluto’s moon Charon came from.
In addition to its compliment of scientific instruments (listed below), there are several cultural artifacts traveling aboard the spacecraft. These include a collection of 434,738 names stored on a compact disc, a piece of Scaled Composites’s SpaceShipOne, and a flag of the USA, along with other mementos. In addition, about 30 g (1 oz) of Clyde Tombaugh’s ashes are aboard the spacecraft, to commemorate his discovery of Pluto in 1930.
The New Horizons spacecraft takes off on Jan. 19, 2006 from the Kennedy Space Center for its planned close encounter with Pluto. Credit: NIKON/Scott Andrews/NASA
Instrumentation:
The New Horizons science payload consists of seven instruments. They are (in alphabetically order):
Alice: An ultraviolet imaging spectrometer responsible for analyzing composition and structure of Pluto’s atmosphere and looks for atmospheres around Charon and Kuiper Belt Objects (KBOs).
LORRI: (Long Range Reconnaissance Imager) a telescopic camera that obtains encounter data at long distances, maps Pluto’s farside and provides high resolution geologic data.
PEPSSI: (Pluto Energetic Particle Spectrometer Science Investigation) an energetic particle spectrometer which measures the composition and density of plasma (ions) escaping from Pluto’s atmosphere.
Ralph: A visible and infrared imager/spectrometer that provides color, composition and thermal maps.
REX: (Radio Science EXperiment) a device that measures atmospheric composition and temperature; passive radiometer.
SDC: (Student Dust Counter) built and operated by students, this instrument measures the space dust peppering New Horizons during its voyage across the solar system.
SWAP: (Solar Wind Around Pluto) a solar wind and plasma spectrometer that measures atmospheric “escape rate” and observes Pluto’s interaction with solar wind.
The instruments New Horizons will use to characterize Pluto. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
Launch:
Due to a series of weather-related delays, the New Horizons mission launched on January 19th, 2006, two days later than originally scheduled. The spacecraft took off from Cape Canaveral Air Force Station, Florida, at 15:00 EST (19:00 UTC) atop an Atlas V 551 rocket. This was the first launch of this particular rocket configuration, which has a third stage added to increase the heliocentric (escape) speed.
The spacecraft left Earth faster than any spacecraft to date, achieving a launch velocity of 16.5 km/s. It took only nine hours to reach the Moon’s orbit, passing lunar orbit before midnight (EST) on the same day it was launched. It has not, however, broken Voyager 1‘s record – which is currently traveling at 17.145 km/s (61,720 km/h, 38,350 mph) relative to the Sun – for being the fastest spacecraft to leave the Solar System.
Inner Solar System:
Between January and March, 2006, mission controllers guided the probe through a series of trajectory-correction maneuvers (TCMs). During the week of February 20th, 2006, controllers conducted in-flight tests on three of the major on board science instruments. On April 7th, the spacecraft passed the orbit of Mars, moving at roughly 21 km/s (76,000 km/h; 47,000 mph) away from the Sun.
At this point in its journey, the spacecraft had reached a distance of 243 million kilometers from the Sun, and approximately 93.4 million km from Earth. On June 13th, 2006, the New Horizons spacecraft passed the tiny asteroid 132524 APL at a distance of 101,867 km (63,297 mi) when it was closest.
Using the Ralph instrument, New Horizons was able to capture images of the asteroid, estimating to be 2.5 km (1.6 mi) in diameter. The spacecraft also successfully tracked the asteroid from June 10th-12th, 2006, allowing the mission team to test the spacecraft’s ability to track rapidly moving objects.
First images of Pluto taken by New Horizons in September 2006. Credit: NASA
From September 21st-24th, New Horizons managed to capture its first images of Pluto while testing the LORRI instruments. These images, which were taken from a distance of approximately 4,200,000,000 km (2.6×109 mi) or 28.07 AU and released on November 28th, confirmed the spacecraft’s ability to track distant targets.
Outer Solar System:
On September 4th, 2006, New Horizons took its first pictures of Jupiter at a distance of 291 million kilometers (181 million miles). The following January, it conducted more detailed surveys of the system, capturing an infrared image of the moon Callisto, and several black and white images of Jupiter itself.
By February 28th, 2007, at 23:17 EST (03:17, UTC) New Horizons made its closest approach to Europa, at a distance of 2,964,860 km (1,842,278 mi). At 01:53:40 EST (05:43:40 UTC), the spacecraft made its flyby of Jupiter, at a distance of 2.3 million km (1.4 million mi) and received a gravity assist.
The Jupiter flyby increased New Horizons‘ speed by 4 km/s (14,000 km/h; 9,000 mph), accelerating the probe to a velocity of 23 km/s (83,000 km/h; 51,000 mph) relative to the Sun and shortening its voyage to Pluto by three years.
The encounter with Jupiter not only provided NASA with the opportunity to photograph the planet using the latest equipment, it also served as a dress rehearsal for the spacecraft’s encounter with Pluto. As well as testing the imaging instruments, it also allowed the mission team to test the communications link and the spacecraft’s memory buffer.
Black and white image of Jupiter viewed by LORRI in January 2007. Credit: NASA/John Hopkins University Applied Physics Laboratory/Southwest Research Institute
One of the main goals during the Jupiter encounter was observing its atmospheric conditions and analyzing the structure and composition of its clouds. Heat-induced lightning strikes in the polar regions and evidence of violent storm activity were both observed. In addition, the Little Red Spot, was imaged from up close for the first time. The New Horizons spacecraft also took detailed images of Jupiter’s faint ring system. Traveling through Jupiter’s magnetosphere, the spacecraft also managed to collect valuable particle readings.
The flyby of the Jovian systems also gave scientists the opportunity to examine the structure and motion of Io’s famous lava plumes. New Horizons measured the plumes coming from the Tvashtar volcano, which reached an altitude of up to 330 km from the surface, while infrared signatures confirmed the presence of 36 more volcanoes on the moon.
Callisto’s surface was also analyzed with LEISA, revealing how lighting and viewing conditions affect infrared spectrum readings of its surface water ice. Data gathered on minor moons such as Amalthea also allowed NASA scientists to refine their orbit solutions.
After passing Jupiter, New Horizons spent most of its journey towards Pluto in hibernation mode. During this time, New Horizons crossed the orbit of Saturn (June 8, 2008) and Uranus on (March 18, 2011). In June 2014, the spacecraft emerged from hibernation and the team began conducting instrument calibrations and a course correction,. By August 24th, 2014, it crossed Neptune’s orbit on its way to Pluto.
New Horizons Long Range Reconnaissance Imager (LORRI) captured these two images of Jupiter’s outermost large moon, Callisto, during its flyby in February 2007. Credit: NASA/JPL
Rendezvous with Pluto:
Distant-encounter operations at Pluto began on January 4th, 2015. Between January 25th to 31st, the approaching probe took several images of Pluto, which were released by NASA on February 12th. These photos, which were taken at a distance of more than 203,000,000 km (126,000,000 mi) showed Pluto and its largest moon, Charon.
Investigators compiled a series of images of the moons Nix and Hydra taken from January 27th through February 8th, 2015, beginning at a range of 201,000,000 km (125,000,000 mi), while Kerberos and Styx were captured by photos taken on April 25.
On July 4th, 2015, NASA lost contact with New Horizons after it experienced a software anomaly and went into safe mode. On the following day, NASA announced that they had determined it to be the result of a timing flaw in a command sequence. By July 6th, the glitch had been fixed and the probe had exited safe mode and began making its approach.
The New Horizons spacecraft made its closest approach to Pluto at 07:49:57 EDT (11:49:57 UTC) on July 14th, 2015, and then Charon at 08:03:50 EDT (12:03:50 UTC). Telemetries confirming a successful flyby and a healthy spacecraft reached Earth on 20:52:37 EDT (00:52:37 UTC).
During the flyby, the probe captured the clearest pictures of Pluto to date, and full analyses of the data obtained is expected to take years to process. The spacecraft is currently traveling at a speed of 14.52 km/s (9.02 mi/s) relative to the Sun and at 13.77 km/s (8.56 mi/s) relative to Pluto.
Full trajectory of New Horizons space probe (sideview). Credit: pluto.jhuapl.edu
Future Objectives:
With its flyby of Pluto now complete, the New Horizons probe is now on its way towards the Kuiper Belt. The goal here is to study one or two other Kuiper Belt Objects, provided suitable KBOs are close to New Horizons‘ flight path.
Three objects have since been selected as potential targets, which were provisionally designated PT1 (“potential target 1”), PT2 and PT3 by the New Horizons team. These have since been re-designated as 2014 MU69 (PT1), 2014 OS393 (PT2), and 2014 PN70 (PT3).
All of these objects have an estimated diameter of 30–55 km, are too small to be seen by ground telescopes, and are 43–44 AU from the Sun, which would put the encounters in the 2018–2019 period. All are members of the “cold” (low-inclination, low-eccentricity) classical Kuiper Belt, and thus very different from Pluto.
Even though it was launched far faster than any outward probe before it, New Horizons will never overtake either Voyager 1 or Voyager 2 as the most distant human-made object from Earth. But then again, it doesn’t need to, given that what it was sent out to study all lies closer to home.
What’s more, the probe has provided astronomers with extensive and updated data on many of planets and moons in our Solar System – not the least of which are the Jovian and Plutonian systems. And last, but certainly not least, New Horizons is the first spacecraft to have it made it out to such a distance since the Voyager program.
And so we say so long and good luck to New Horizons, not to mention thanks for providing us with the best images of Pluto anyone has ever seen! We can only hope she fares well as she makes its way into the Kuiper Belt and advances our knowledge of the outer Solar System even farther.
Distribution of Kuiper belt objects (green), along with various other outer Solar System bodies, based on data from the Minor Planet Center. [Credit: Minor Planet Center; Murray and Dermott]
The Kuiper belt — the region beyond the orbit of Neptune inhabited by a number of small bodies of rock and ice — hides many clues about the early days of the Solar System. According to the standard picture of Solar System formation, many planetesimals were born in the chaotic region where the giant planets now reside. Some were thrown out beyond the orbit of Neptune, while others stayed put in the form of Trojan asteroids (which orbit in the same trajectory as Jupiter and other planets). This is called the Nice model.
However, not all Kuiper belt objects (KBOs) play nicely with the Nice model.
(I should point out that the model is named named for the city in France and therefore pronounced “neese”.) A new study of large scale surveys of KBOs revealed that those with nearly circular orbits lying roughly in the same plane as the orbits of the major planets don’t fit the Nice model, while those with irregular orbits do. It’s a puzzling anomaly, one with no immediate resolution, but it hints that we need to refine our Solar System formation models.
This new study is described in a recently released paper by Wesley Fraser, Mike Brown, Alessandro Morbidelli, Alex Parker, and Konstantin Baygin (to be published in the Astrophysical Journal, available online). These researchers combined data from seven different surveys of KBOs to determine roughly how many of each size of object are in the Solar System, which in turn is a good gauge of the environment in which they formed.
The difference between this and previous studies is the use of absolute magnitudes — a measure of how bright an object really is — as opposed to their apparent magnitudes, which are simply how bright an object appears. The two types of magnitude are related by the distance an object is from Earth, so the observational challenge comes down to accurate distance measurements. Absolute magnitude is also related to the size of an KBO and its albedo (how much light it reflects), both important physical quantities for understanding formation and composition.
Finding the absolute magnitudes for KBOs is more challenging than apparent magnitudes for obvious reasons: these are small objects, often not resolved as anything other than points of light in a telescope. That means requires measuring the distance to each KBO as accurately as possible. As the authors of the study point out, even small errors in distance measurements can have a large effect on the estimated absolute magnitude.
The bodies in the Kuiper Belt. Credit: Don Dixon
In terms of orbits, KBOs fall into two categories: “hot” and “cold”, confusing terms having nothing to do with temperature. The “cold” KBOs are those with nearly circular orbits (low eccentricity, in mathematical terms) and low inclinations, meaning their trajectories lie nearly in the ecliptic plane, where the eight canonical planets also orbit. In other words, these objects have nearly planet-like orbits. The “hot” KBOs have elongated orbits and higher inclinations, behavior more akin to comets.
The authors of the new study found that the hot KBOs have the same distribution of sizes as the Trojan asteroids, meaning there are the same relative number of small, medium, and large KBOs and similarly sized Trojans. That hints at a probable common origin in the early days of the Solar System. This is in line with the Nice model, which predicts that, as they migrated into their current orbits, the giant planets kicked many planetesimals out beyond Neptune.
However, the cold KBOs don’t match that pattern at all: there are fewer large KBOs relative to smaller objects. To make matters more strange, both hot and cold seem to follow the same pattern for the smaller bodies, only deviating at larger masses, which is at odds with expectations if the cold KBOs formed where they orbit today.
To put it another way, the Nice model as it stands could explain the hot KBOs and Trojans, but not the cold. That doesn’t mean all is lost, of course. The Nice model seems to do very well except for a few nagging problems, so it’s unlikely that it’s completely wrong. As we’ve learned from studying exoplanet systems, planet formation models are a work in progress — and astronomers are an ingenious lot.
