Illustration of NASA's Transiting Exoplanet Survey Satellite. Credit: NASA's Goddard Space Flight Center
NASA’s Transiting Exoplanet Survey Satellite, or TESS, was designed to find other worlds. Following in the tradition of the Kepler spacecraft, TESS has a hundred thousand stars looking for small but regular dips in their brightness. These dips are typically caused by planets as they pass in front of the star. TESS has been quite effective, logging nearly 6,000 candidate exoplanets. Confirming or rejecting these candidates takes time, but it has led to some interesting discoveries.
Artist rendition of a potential water-world exoplanet that might support life. Scientists could determine whether to explore this world based on its planetary entropy production. (Credit: ESA / Hubble / M. Kornmesser)
In a recent study submitted to The Astrophysical Journal Letters, an international team of researchers led by the University of California, Los Angeles (UCLA) examine the potential for water-worlds around M-dwarf stars. Water-worlds, also known as ocean worlds, are planets that possess bodies of liquid water either directly on its surface, such as Earth, or somewhere beneath it, such as Jupiter’s moon, Europa and Saturn’s moon, Enceladus.
This artist's illustration shows what an icy exo-Earth might look like. A new study says liquid water could persist under ice sheets on planets outside of their habitable zones. Image Credit: NASA
Our understanding of habitability relies entirely on the availability of liquid water. All life on Earth needs it, and there’s every indication that life elsewhere needs it, too.
Can planets with frozen surfaces somehow have enough water to sustain life?
In a series of papers, Professor Loeb and Michael Hippke indicate that conventional rockets would have a hard time escaping from certain kinds of extra-solar planets. Credit: NASA/Tim Pyle
Since the beginning of the Space Age, humans have relied on chemical rockets to get into space. While this method is certainly effective, it is also very expensive and requires a considerable amount of resources. As we look to more efficient means of getting out into space, one has to wonder if similarly-advanced species on other planets (where conditions would be different) would rely on similar methods.
Harvard Professor Abraham Loeb and Michael Hippke, an independent researcher affiliated with the Sonneberg Observatory, both addressed this question in two recently–released papers. Whereas Prof. Loeb looks at the challenges extra-terrestrials would face launching rockets from Proxima b, Hippke considers whether aliens living on a Super-Earth would be able to get into space.
Artist’s impression of Proxima b, which was discovered using the Radial Velocity method. Credit: ESO/M. Kornmesser
For the sake of his study, Loeb considered how we humans are fortunate enough to live on a planet that is well-suited for space launches. Essentially, if a rocket is to escape from the Earth’s surface and reach space, it needs to achieve an escape velocity of 11.186 km/s (40,270 km/h; 25,020 mph). Similarly, the escape velocity needed to get away from the location of the Earth around the Sun is about 42 km/s (151,200 km/h; 93,951 mph).
As Prof. Loeb told Universe Today via email:
“Chemical propulsion requires a fuel mass that grows exponentially with terminal speed. By a fortunate coincidence the escape speed from the orbit of the Earth around the Sun is at the limit of attainable speed by chemical rockets. But the habitable zone around fainter stars is closer in, making it much more challenging for chemical rockets to escape from the deeper gravitational pit there.”
As Loeb indicates in his essay, the escape speed scales as the square root of the stellar mass over the distance from the star, which implies that the escape speed from the habitable zone scales inversely with stellar mass to the power of one quarter. For planets like Earth, orbiting within the habitable zone of a G-type (yellow dwarf) star like our Sun, this works out quite while.
This infographic compares the orbit of the planet around Proxima Centauri (Proxima b) with the same region of the Solar System. Credit: Pale Red Dot
Unfortunately, this does not work well for terrestrial planets that orbit lower-mass M-type (red dwarf) stars. These stars are the most common type in the Universe, accounting for 75% of stars in the Milky Way Galaxy alone. In addition, recent exoplanet surveys have discovered a plethora of rocky planets orbiting red dwarf stars systems, with some scientists venturing that they are the most likely place to find potentially-habitable rocky planets.
Using the nearest star to our own as an example (Proxima Centauri), Loeb explains how a rocket using chemical propellant would have a much harder time achieving escape velocity from a planet located within it’s habitable zone.
“The nearest star to the Sun, Proxima Centauri, is an example for a faint star with only 12% of the mass of the Sun,” he said. “A couple of years ago, it was discovered that this star has an Earth-size planet, Proxima b, in its habitable zone, which is 20 times closer than the separation of the Earth from the Sun. At that location, the escape speed is 50% larger than from the orbit of the Earth around the Sun. A civilization on Proxima b will find it difficult to escape from their location to interstellar space with chemical rockets.”
Hippke’s paper, on the other hand, begins by considering that Earth may in fact not be the most habitable type of planet in our Universe. For instance, planets that are more massive than Earth would have higher surface gravity, which means they would be able to hold onto a thicker atmosphere, which would provide greater shielding against harmful cosmic rays and solar radiation.
Artists impression of a Super-Earth, a class of planet that has many times the mass of Earth, but less than a Uranus or Neptune-sized planet. Credit: NASA/Ames/JPL-Caltech
In addition, a planet with higher gravity would have a flatter topography, resulting in archipelagos instead of continents and shallower oceans – an ideal situation where biodiversity is concerned. However, when it comes to rocket launches, increased surface gravity would also mean a higher escape velocity. As Hippke indicated in his study:
“Rockets suffer from the Tsiolkovsky (1903) equation : if a rocket carries its own fuel, the ratio of total rocket mass versus final velocity is an exponential function, making high speeds (or heavy payloads) increasingly expensive.”
For comparison, Hippke uses Kepler-20 b, a Super-Earth located 950 light years away that is 1.6 times Earth’s radius and 9.7 times it mass. Whereas escape velocity from Earth is roughly 11 km/s, a rocket attempting to leave a Super-Earth similar to Kepler-20 b would need to achieve an escape velocity of ~27.1 km/s. As a result, a single-stage rocket on Kepler-20 b would have to burn 104 times as much fuel as a rocket on Earth to get into orbit.
To put it into perspective, Hippke considers specific payloads being launched from Earth. “To lift a more useful payload of 6.2 t as required for the James Webb Space Telescope on Kepler-20 b, the fuel mass would increase to 55,000 t, about the mass of the largest ocean battleships,” he writes. “For a classical Apollo moon mission (45 t), the rocket would need to be considerably larger, ~400,000 t.”
Project Starshot, an initiative sponsored by the Breakthrough Foundation, is intended to be humanity’s first interstellar voyage. Credit: breakthroughinitiatives.org
While Hippke’s analysis concludes that chemical rockets would still allow for escape velocities on Super-Earths up to 10 Earth masses, the amount of propellant needed makes this method impractical. As Hippke pointed out, this could have a serious effect on an alien civilization’s development.
“I am surprised to see how close we as humans are to end up on a planet which is still reasonably lightweight to conduct space flight,” he said. “Other civilizations, if they exist, might not be as lucky. On more massive planets, space flight would be exponentially more expensive. Such civilizations would not have satellite TV, a moon mission, or a Hubble Space Telescope. This should alter their way of development in certain ways we can now analyze in more detail.”
Both of these papers present some clear implications when it comes to the search for extra-terrestrial intelligence (SETI). For starters, it means that civilizations on planets that orbit red dwarf stars or Super-Earths are less likely to be space-faring, which would make detecting them more difficult. It also indicates that when it comes to the kinds of propulsion humanity is familiar with, we may be in the minority.
“This above results imply that chemical propulsion has a limited utility, so it would make sense to search for signals associated with lightsails or nuclear engines, especially near dwarf stars,” said Loeb. “But there are also interesting implications for the future of our own civilization.”
Artist’s concept of a bimodal nuclear rocket making the journey to the Moon, Mars, and other destinations in the Solar System. Credit: NASA
“One consequence of the paper is for space colonization and SETI,” added Hippke. “Civs from Super-Earths are much less likely to explore the stars. Instead, they would be (to some extent) “arrested” on their home planet, and e.g. make more use of lasers or radio telescopes for interstellar communication instead of sending probes or spaceships.”
However, both Loeb and Hippke also note that extra-terrestrial civilizations could address these challenges by adopting other methods of propulsion. In the end, chemical propulsion may be something that few technologically-advanced species would adopt because it is simply not practical for them. As Loeb explained:
“An advanced extraterrestrial civilization could use other propulsion methods, such as nuclear engines or lightsails which are not constrained by the same limitations as chemical propulsion and can reach speeds as high as a tenth of the speed of light. Our civilization is currently developing these alternative propulsion technologies but these efforts are still at their infancy.”
One such example is Breakthrough Starshot, which is currently being developed by the Breakthrough Prize Foundation (of which Loeb is the chair of the Advisory Committee). This initiative aims to use a laser-driven lightsail to accelerate a nanocraft up to speeds of 20% the speed of light, which will allow it to travel to Proxima Centauri in just 20 years time.
Artist’s impression of rocky exoplanets orbiting Gliese 832, a red dwarf star just 16 light-years from Earth. Credit: ESO/M. Kornmesser/N. Risinger (skysurvey.org).
Hippke similarly considers nuclear rockets as a viable possibility, since increased surface gravity would also mean that space elevators would be impractical. Loeb also indicated that the limitations imposed by planets around low mass stars could have repercussions for when humans try to colonize the known Universe:
“When the sun will heat up enough to boil all water off the face of the Earth, we could relocate to a new home by then. Some of the most desirable destinations would be systems of multiple planets around low mass stars, such as the nearby dwarf star TRAPPIST-1 which weighs 9% of a solar mass and hosts seven Earth-size planets. Once we get to the habitable zone of TRAPPIST-1, however, there would be no rush to escape. Such stars burn hydrogen so slowly that they could keep us warm for ten trillion years, about a thousand times longer than the lifetime of the sun.”
But in the meantime, we can rest easy in the knowledge that we live on a habitable planet around a yellow dwarf star, which affords us not only life, but the ability to get out into space and explore. As always, when it comes to searching for signs of extra-terrestrial life in our Universe, we humans are forced to take the “low hanging fruit approach”.
Basically, the only planet we know of that supports life is Earth, and the only means of space exploration we know how to look for are the ones we ourselves have tried and tested. As a result, we are somewhat limited when it comes to looking for biosignatures (i.e. planets with liquid water, oxygen and nitrogen atmospheres, etc.) or technosignatures (i.e. radio transmissions, chemical rockets, etc.).
As our understanding of what conditions life can emerge under increases, and our own technology advances, we’ll have more to be on the lookout for. And hopefully, despite the additional challenges it may be facing, extra-terrestrial life will be looking for us!
Artist’s impression of a view from the HD 7924 planetary system looking back toward our sun, which would be easily visible to the naked eye. Since HD 7924 is in our northern sky, an observer looking back at the sun would see objects like the Southern Cross and the Magellanic Clouds close to our sun in their sky. Credit: Karen Termaura and BJ Fulton, UH IfA
As of March 1st, 2018, 3,741 exoplanets have been confirmed in 2,794 systems, with 622 systems having more than one planet. Most of the credit for these discoveries goes to the Kepler space telescope, which has discovered roughly 3500 planets and 4500 planetary candidates. In the wake of all these discoveries, the focus has shifted from pure discovery to research and characterization.
In this respect, planets detected using the Transit Method are especially valuable since they allow for the study of these planets in detail. For example, a team of astronomers recently discovered three Super-Earths orbiting a star known GJ 9827, which is located just 100 light years (30 parsecs) from Earth. The proximity of the star, and the fact that it is orbited by multiple Super-Earths, makes this system ideal for detailed exoplanet studies.
Artistic design of the super-Earth GJ 625 b and its star, GJ625 (Gliese 625). Credit: Gabriel Pérez/SMM (IAC)
As with all Kepler discoveries, these planets were discovered using the Transit Method (aka. Transit Photometry), where stars are monitored for periodic dips of brightness. These dips are the result of exoplanets passing in front of the star (i.e. transiting) relative to the observer. While this method is ideal for placing constraints on the size and orbital periods of a planet, it can also allow for exoplanet characterization.
Basically, scientists are able to learn things about their atmospheres by measuring the spectra produced by the star’s light as it passes through the planet’s atmosphere. Combined with radial velocity measurements of the star, scientists can also place constraints on the planet’s mass and radius and can determine things about the planet’s interior structure.
For the sake of their study, the team analyzed data obtained by the K2 mission, which showed the presence of three Super-Earths around the star GJ 9827 (GJ 9827 b, c, and d). Since they initially submitted their research paper back in September of 2017, the presence of these planets has been confirmed by another team of astronomers. As Dr. Rodriguez told Universe Today via email:
“We detected three super-Earth sized planets orbiting in a very compact configuration. Specifically, the three planets have radii of 1.6, 1.2, and 2.1 times the radius of Earth and all orbit their host star within 6.2 days. We note that this system was independently discovered (simultaneously) by another team from Wesleyan University (Niraula et al. 2017).”
The super-Earth exoplanet 55 Cancri e, depicted with its star in this artist’s concept, likely has an atmosphere thicker than Earth’s but with ingredients that could be similar to those of Earth’s atmosphere. Credit: NASA/JPL
These three exoplanets are especially interesting because the larger of the two have radii that place them in the range between being rocky or gaseous. Few such exoplanets have been discovered so far, which makes these three a prime target for research. As Dr. Rodriguez explained:
“Super Earth sized planets are the most common type of planet we know of but we do not have one in our own solar system, limiting our ability to understand them. They are especially important because their radii span the rock to gas transition (as I discuss below in one of the other responses). Essentially, planets larger then 1.6 times the radius of the Earth are less dense and have thick hydrogen/helium atmospheres while planets smaller are very dense with little to no atmosphere.”
Another interesting thing about these super-Earths is how their short orbital periods – which are 1.2, 3.6 and 6.2 days, respectively – would result in fairly hot temperatures. In short, the team estimates that the three super-Earths experience surface temperatures of 1172 K (899 °C; 1650 °F), 811 K (538 °C; 1000 °F), and 680 K (407 °C; 764 °F), respectively.
By comparison, Venus – the hottest planet in the Solar System – experiences surface temperatures of 735 K (462 °C; 863 °F). So while temperatures on Venus are hot enough to melt lead, conditions on GJ 9827 b are almost hot enough to melt bronze.
The light curve obtained during Campaign 12 of the K2 mission of the GJ 9827 system. Credit: Rodriguez et al., 2018.
However, the most significant thing about this discovery is the opportunities it could provide for exoplanet characterization. At just 100 light-years from Earth, it will be relatively easy for the next-generation telescopes (such as the James Webb Space Telescope) to conduct studies of their atmospheres and provide a more detailed picture of this system of planets.
In addition, these three strange planets are all in the same system, which makes conducting observation campaigns that much easier. As Rodriguez concluded:
“The GJ 9827 system is unique because one planet is smaller than this cutoff, one planet is larger, and the third planet has a radius of ~1.6 times the radius of the Earth, right on that border. So in one system, we have planets that span this rock to gas transition. This is important because we can study the atmosphere’s of these planets, look for differences in the composition of their atmospheres and begin to understand why this transition occurs at 1.6 times the radius of the Earth. Since all three planets orbit the same star, the effect of the host star is kept constant in this “experiment”. Therefore, if these three planets in GJ 9827 were instead orbiting three separate stars, we would have to worry about how the host star is influencing or affecting the planet’s atmosphere. In the GJ 9827 system, we do not have to worry about this since they orbit the same star.”
K2-18b and its neighbour, newly discovered K2-18c, orbit the red-dwarf star k2-18 locataed 111 light years away in the constellation Leo. Credit: Alex Boersma
The search for extra-solar planets has turned up some very interesting discoveries. Aside planets that are more-massive versions of their Solar counterparts (aka. Super-Jupiters and Super-Earths), there have been plenty of planets that straddle the line between classifications. And then there were times when follow-up observations have led to the discovery of multiple planetary systems.
This was certainly the case when it came to K2-18, a red dwarf star system located about 111 light-years from Earth in the constellation Leo. Using the ESO’s High Accuracy Radial Velocity Planet Searcher (HARPS), an international team of astronomers was recently examining a previously-discovered exoplanet in this system (K2-18b) when they noted the existence of a second exoplanet.
Artist’s impression of a Super-Earth planet orbiting a Sun-like star. Credit: ESO/M. Kornmesser
Led by Ryan Cloutier, a PhD student at the University of Toronto’s Center for Planet Science and the University of Montréal’s Institute for Research on Exoplanets (iREx), the team included members from the University of Geneva, the University Grenoble Alpes, and the University of Porto. Together, the team conducted a study of K2-18b in the hopes of characterizing this exoplanet and determining its true nature.
When K2-18b was first discovered in 2015, it was found to be orbiting within the star’s habitable zone (aka. “Goldilocks Zone“). The team responsible for the discovery also determined that given its distance from its star, K2-18b’s surface received similar amounts of radiation as Earth. However, the initial estimates of the planet’s size left astronomers uncertain as to whether the planet was a Super-Earth or a mini-Neptune.
For this reason, Cloutier and his team sought to characterize the planet’s mass, a necessary step towards determining it’s atmospheric properties and bulk composition. To this end, they obtained radial velocity measurements of K2-18 using the HARPS spectrograph. These measurements allowed them to place mass constraints on previously-discovered exoplanet, but also revealed something extra.
“Being able to measure the mass and density of K2-18b was tremendous, but to discover a new exoplanet was lucky and equally exciting… If you can get the mass and radius, you can measure the bulk density of the planet and that can tell you what the bulk of the planet is made of.”
Artist’s impression of a super-Earth with a dense atmosphere, which is what scientists now believe K2-18b is. Credit: NASA/JPL
Essentially, their radial velocity measurements revealed that K2-18b has a mass of about 8.0 ± 1.9 Earth masses and a bulk density of 3.3 ± 1.2 g/cm³. This is consistent with a terrestrial (aka. rocky) planet with a significant gaseous envelop and a water mass fraction that is equal to or less than 50%. In other words, it is either a Super-Earth with a small gaseous atmosphere (like Earths) or “water world” with a thick layer of ice on top.
They also found evidence for a second “warm” Super Earth named K2-18c, which has a mass of 7.5 ± 1.3 Earth masses, an orbital period of 9 days, and a semi-major axis roughly 2.4 times smaller than K2-18b. After re-examining the original light curves obtained from K2-18, they concluded that K2-18c was not detected because it has an orbit that does not lie on the same plane. As Cloutier described the discovery:
“When we first threw the data on the table we were trying to figure out what it was. You have to ensure the signal isn’t just noise, and you need to do careful analysis to verify it, but seeing that initial signal was a good indication there was another planet… It wasn’t a eureka moment because we still had to go through a checklist of things to do in order to verify the data. Once all the boxes were checked it sunk in that, wow, this actually is a planet.”
Unfortunately, the newly-discovered K2-18c orbits too closely to its star for it to be within it’s habitable zone. However, the likelihood of K2-18b being habitable remains probable, thought that depends on its bulk composition. In the end, this system will benefit from additional surveys that will more than likely involve NASA’s James Webb Space Telescope (JWST) – which is scheduled for launch in 2019.
Artist’s impression of Super-Earth orbiting closely to its red dwarf star. Credit: M. Weiss/CfA
These surveys are expecting to resolve the latest mysteries about this planet, which is whether it is Earth-like or a “water world”. “With the current data, we can’t distinguish between those two possibilities,” said Cloutier. “But with the James Webb Space Telescope (JWST) we can probe the atmosphere and see whether it has an extensive atmosphere or it’s a planet covered in water.”
“There’s a lot of demand to use this telescope, so you have to be meticulous in choosing which exoplanets to look at. K2-18b is now one of the best targets for atmospheric study, it’s going to the near top of the list.”
The discovery of this second Super-Earth in the K2-18 system is yet another indication of how prevalent multi-planet systems are around M-type (red dwarf) stars. The proximity of this system, which has at least one planet with a thick atmosphere, also makes it well-suited to studies that will teach astronomers more about the nature of exoplanet atmospheres.
Expect to hear more about this star and its planetary system in the coming years!
Artists impression of a Super-Earth, a class of planet that has many times the mass of Earth, but less than a Uranus or Neptune-sized planet. Credit: NASA/Ames/JPL-Caltech
Red dwarf stars have proven to be a treasure trove for exoplanet hunters in recent years. In addition to multiple exoplanets candidates being detected around stars like TRAPPIST-1, Gliese 581, Gliese 667C, and Kepler 296, there was also the ESO’s recent discovery of a planet orbiting within the habitable zone of our Sun’s closest neighbor – Proxima Centauri.
And it seems the trend is likely to continue, with the latest discovery comes from a team of European scientists. Using data from the ESO’s High Accuracy Radial velocity Planet Searcher (HARPS) and HARPS-N instruments, they detected an exoplanet candidate orbiting around GJ 536 – an M-class red dwarf star located about 32.7 light years (10.03 parsecs) from Earth.
According to their study, “A super-Earth Orbiting the Nearby M-dwarf GJ 536“, this planet is a super-Earth – a class of exoplanet that has between more than one, but less than 15, times the mass of Earth. In this case, the planet boasts a minimum of 5.36 ± 0.69 Earth masses, has an orbital period of 8.7076 ± 0.0025 days, and orbits its sun at a distance of 0.06661 AU.
Artist’s impression of a system of exoplanets orbiting a low mass, red dwarf star. Credit: NASA/JPL
The team was led by Dr. Alejandro Suárez Mascareño of the Instituto de Astrofísica de Canarias (IAC). The discovery of the planet was part of his thesis work, which was conducted under Dr Rafael Rebolo – who is also a member of the IAC, the Spanish National Research Council and a professor at the University of Laguna. And while the planet is not a potentially habitable world, it does present some interesting opportunities for exoplanet research.
As Dr. Mascareño shared with Universe Today via email:
“GJ 536 b is a small super Earth discovered in a very nearby star. It is part of the group of the smallest planets with measured mass. It is not in the habitable zone of its star, but its relatively close orbit and the brightness of its star makes it a promising target for transmission spectroscopy IF we can detect the transit. With a star so bright (V 9.7) it would be possible to obtain good quality spectra during the hypothetical transit to try to detect elements in the atmosphere of the planet. We are already designing a campaign for next year, but I guess we won’t be the only ones.”
The survey that found this planet was part of a joint effort between the IAC (Spain) and the Geneva Observatory (Switzerland). The data came from the HARPS and HARPS-N instruments, which are mounted on the ESO’s 3.6 meter telescope at the La Silla Observstory in Chile and the 3.6 meter telescope at the La Palma Observatory in Spain. This was combined with photometric data from the All Sky Automated Survey (ASAS), which has observatories in Chile and Maui.
The research team relied on radial velocity measurements from the star to discern the presence of the planet, as well as spectroscopic observations of the star that were taken over a 8.6 year period. For all this, they not only detected an exoplanet candidate with 5 times the mass of Earth, but also derived information on the star itself – which showed that it has a rotational period of about 44 days, and magnetic cycle that lasts less than three years.
Artist’s depiction of the interior of a low-mass star, such as the one seen in an X-ray image from Chandra in the inset. Credit: NASA/CXC/M.Weiss
By comparison, our Sun has a rotational period of 25 days and a magnetic cycle of 11 years, which is characterized by changes in the levels of solar radiation it emits, the ejection of solar material and in the appearance of sunspots. In addition, a recent study from the the Harvard Smithsonian Center for Astrophysics (CfA) showed that Proxima Centauri has a stellar magnetic cycle that lasts for 7 years.
This detection is just the latest in a long line of exoplanets being discovered around low-mass, low-luminosity, M-class (red dwarf) stars. And looking ahead, the team hopes to continue surveying GJ 536 to see if there is a planetary system, which could include some Earth-like planets, and maybe even a few gas giants.
“For now we have detected only one planet, but we plan to continue monitoring the star to search for other companions at larger orbital separations,” said Dr. Mascareño. “We estimate there is still room for other low-mass or even Neptune-mass planets at orbits from a hundred of days to a few years.”
The research also included scientists from the Astronomical Observatory at the University of Geneva, the University of Grenoble, The Astrophysical and Planetological Insitute of Grenoble, Institute of Astrophysics and Space Sciences in Portugal, and the University of Porto, Portugal.
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One of the first known stars to host an extrasolar planet, was that of 55 Cancri. The first planet in this system was reported in 1997 and today the system is known to host at least five planets, the inner most of which, 55 Cnc e, was recently discovered to transit the star, giving new information about this planet.
55 Cnc is an interesting system in many respects. Being a mere 41 lightyears from the Earth, the system is composed of a primary, yellow dwarf star in a wide binary orbit (1,000 AU) with a red dwarf. The planetary system lies within this orbit. The primary star is just brighter than 6th magnitude meaning it is visible to the naked eye under good viewing conditions.
One of these planets, 55 Cnc e, was discovered in this system via radial velocity measurements in 2004. At that point, the planet was reported to have a period of 2.8 days, and a minimum mass of 14.2 times the mass of the Earth. However, in 2010, Rebekah Dawson and Daniel Fabrycky from the Harvard-Smithsonian Center for Astrophysics argued that gaps in the observational period skewed the statistics and the true period the planet should be a short 0.7365 days.
One of the results of this was that the planet would have to orbit closer to the parent star. In turn, this increased the likelihood that the planet could transit the star from 13% to 33%. A team led by Joshua Winn from the Massachusetts Institute of Technology went searching for this faint transit and report its detection in a recent paper. But while the star itself is one of the brightest stars in our sky to harbor known extrasolar planets, the eclipse is far from visible without precise observations, changing by only 0.0002%, one of the smallest changes known. The timing of the eclipses confirms that correction by Dawson and Fabrycky and adds new information about the body.
Given the radius determined as well as the mass, the team was able to estimate the structure of the planet and report that the mass is 8.57 ± 0.64 Earth masses. The reported radius is 1.63 ± 0.16 times that of Earth, and the density is 10.9 ± 3.1 g cm-3 (the average density of Earth is 5.515 g cm-3). This places the planet firmly into the categories of a rocky super-Earth.
The team also explores whether or not the planet could retain an atmosphere in such a close orbit (only three times the radius of the star itself). At this close range, the planet would likely be tidally locked and with an albedo typical of rocky planets, the planet would likely have an average temperature of nearly 2970 K (5,000° F). If the planet were able to redistribute the heat, it may be as low as 2100 K (3,300° F). Either way, a planet of such mass would have difficulty retaining any primordial, gaseous atmosphere. However, the team reports that it may be possible for volcanic activity to create a thin atmosphere of high molecular weight components.
While this new report adds precious little in the grand scheme of the rapidly growing body of knowledge of exoplanets, the authors close with the note that, “there is some pleasure in being able to point to a naked-eye star and know the mass and radius of one of its planets.”
Illustration of WASP-12b in orbit about its host star (Credit: ESA/C Carreau)
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WASP-12b, discovered in 2008, is a real outlier among the 400 or so exoplanets discovered to date. Not that it’s particularly massive (it’s a gas giant, not unlike Jupiter), nor that its homesun (host star) is particularly unusual (it’s rather similar to our own Sun), but it orbits very close to its homesun, and is considerably larger than any other gas giant discovered to date.
Results from recent research explain why WASP-12b is so unusual; we’re watching it die a painful death at the hands of its homesun, which is snacking on it.
“This is the first time that astronomers are witnessing the ongoing disruption and death march of a planet,” says UC Santa Cruz professor Douglas N.C. Lin. Lin is a co-author of the new study and the founding director of the Kavli Institute for Astronomy and Astrophysics (KIAA) at Peking University, which was deeply involved with the research.
The research was led by Shu-lin Li of the National Astronomical Observatories of China. A graduate of KIAA, Li and a research team analyzed observational data on the planet to show how the gravity of its parent star is both inflating its size and spurring its rapid dissolution.
WASP-12b, like most known exoplanets discovered to date, is large and gaseous, resembling Jupiter and Saturn; however, unlike Jupiter, Saturn, or most other exoplanets, it orbits its homesun at extremely close range – 75 times closer than the Earth is to the Sun, or just over 1.5 million km. It is also larger than astrophysical models predict. Its mass is estimated to be almost 50% larger than Jupiter’s and it is 80% larger, giving it six times Jupiter’s volume. It is also unusually toasty, with a daytime temperature of more than 2500° C.
Some mechanism must be responsible for expanding this planet to such an unexpected size, say the researchers. They have focused their analysis on tidal forces, which they say are strong enough to produce the effects observed on WASP-12b.
On Earth, tidal forces between the Earth and the Moon cause local sea levels rise and fall, modestly, twice a day. WASP-12b, however, is so close to its homesun that the gravitational forces are enormous. The tremendous tidal forces acting on the planet completely change the shape of the planet into something similar to that of a rugby or American football.
These tides not only distort the shape of WASP-12b. By continuously deforming the planet, they also create friction in its interior. The friction produces heat, which causes the planet to expand. “This is the first time that there is direct evidence that internal heating (or ‘tidal heating’) is responsible for puffing up the planet to its current size,” says Lin.
Huge as it is, WASP-12b faces an early demise, say the researchers. In fact, its size is part of its problem. It has ballooned to such a point that it cannot retain its mass against the pull of its homesun’s gravity. As the study’s lead author Li explains, “WASP-12b is losing its mass to the host star at a tremendous rate of six billion metric tons each second. At this rate, the planet will be completely destroyed by its host star in about ten million years. This may sound like a long time, but for astronomers it’s nothing. This planet will live less than 500 times less than the current age of the Earth.” The WASP-12 system (Courtesy: KIAA/Graphic: Neil Miller) About this image: The massive gas giant WASP-12b is shown in purple with the transparent region representing its atmosphere. The gas giant planet’s orbit is somewhat non-circular. This indicates that there is probably an unseen lower mass planet in the system, shown in brown, that is perturbing the larger planet’s orbit. Mass from the gas giant’s atmosphere is pulled off and forms a disk around the star, shown in red.
The material that is stripped off WASP-12b does not fall directly onto the parent star; instead it forms a disk around the star and slowly spirals inwards. A careful analysis of the orbital motion of WASP-12b suggests circumstantial evidence of the gravitational force of a second, lower-mass planet in the disk. This planet is most likely a massive version of the Earth – a so-called “super-Earth.”
The disk of planetary material and the embedded super-Earth should be detectable with currently available telescope facilities. Their properties can be used to further constrain the history and fate of the mysterious planet WASP-12b.
In addition to KIAA, support for the WASP-12b research came from NASA, the Jet Propulsion Laboratory, and the National Science Foundation. Along with Li and Lin, co-authors include UC Santa Cruz professor Jonathan Fortney and Neil Miller, a graduate student at the university.
