An artist’s conception shows the planet TRAPPIST-1h. (NASA / JPL-Caltech)
The announcement of a seven-planet system around the star TRAPPIST-1 earlier this year set off a flurry of scientific interest. Not only was this one of the largest batches of planets to be discovered around a single star, the fact that all seven were shown to be terrestrial (rocky) in nature was highly encouraging. Even more encouraging was the fact that three of these planets were found to be orbiting with the star’s habitable zone.
Since that time, astronomers have been seeking to learn all they can about this system of planets. Aside from whether or not they have atmospheres, astronomers are also looking to learn more about their orbits and surface conditions. Thanks to the efforts of a University of Washington-led international team of astronomers, we now have an accurate idea of what conditions might be like on its outermost planet – TRAPPIST-1h.
An artist's illustration of a 16 meter segmented mirror space telescope. There are no actual images of LUVOIR because the design hasn't been finalized yet. Image: Northrop Grumman Aerospace Systems & NASA/STScI
We humans have an insatiable hunger to understand the Universe. As Carl Sagan said, “Understanding is Ecstasy.” But to understand the Universe, we need better and better ways to observe it. And that means one thing: big, huge, enormous telescopes.
In this series we’ll look at the world’s upcoming Super Telescopes:
The Large UV Optical Infrared Surveyor Telescope (LUVOIR)
There’s a whole generation of people who grew up with images from the Hubble Space Telescope. Not just in magazines, but on the internet, and on YouTube. But within another generation or two, the Hubble itself will seem quaint, and watershed events of our times, like the Moon Landing, will be just black and white relics of an impossibly distant time. The next generations will be fed a steady diet of images and discoveries stemming from the Super Telescopes. And the LUVOIR will be front and centre among those ‘scopes.
If you haven’t yet heard of LUVOIR, it’s understandable; LUVOIR is in the early stages of being defined and designed. But LUVOIR represents the next generation of space telescopes, and its power will dwarf that of its predecessor, the Hubble.
LUVOIR (its temporary name) will be a space telescope, and it will do its work at the LaGrange 2 point, the same place that JWST will be. L2 is a natural location for space telescopes. At the heart of LUVOIR will be a 15m segmented primary mirror, much larger than the Hubble’s mirror, which is a mere 2.4m in diameter. In fact, LUVOIR will be so large that the Hubble could drive right through the hole in the center of it.
This not-to-scale image of the Solar System shows the LaGrangian points. LUVOIR will be located in a halo orbit at L2, along with the JWST. Image: By Xander89 – File:Lagrange_points2.svg, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=36697081
While the James Webb Space Telescope will be in operation much sooner than LUVOIR, and will also do amazing work, it will observe primarily in the infrared. LUVOIR, as its name makes clear, will have a wider range of observation more like Hubble’s. It will see in the Ultra-Violet spectrum, the Optical spectrum, and the Infrared spectrum.
Recently, Brad Peterson spoke with Fraser Cain on a weekly Space Hangout, where he outlined the plans for the LUVOIR. Brad is a recently retired Professor of Astronomy at the Ohio State University, where served as chair of the Astronomy Department for 9 years. He is currently the chair of the Science Committee at NASA’s Advisory Council. Peterson is also a Distinguished Visiting Astronomer at the Space Telescope Science Institute, and the chair of the astronomy section of the American Association for the Advancement of Science.
Different designs for LUVOIR have been discussed, but as Peterson points out in the interview above, the plan seems to have settled on a 15m segmented mirror. A 15m mirror is larger than any optical light telescope we have on Earth, though the Thirty Meter Telescope and others will soon be larger.
“Segmented telescopes are the technology of today when it comes to ground-based telescopes. The JWST has taken that technology into space, and the LUVOIR will take segmented design one step further,” Peterson said. But the segmented design of LUVOIR differs from the JWST in several ways.
“…the LUVOIR will take segmented design one step further.” – Brad Peterson
JWST’s mirrors are made of beryllium and coated with gold. LUVOIR doesn’t require the same exotic design. But it has other requirements that will push the envelope of segmented telescope design. LUVOIR will have a huge array of CCD sensors that will require an enormous amount of electrical power to operate.
The Hubble Space Telescope on the left has a 2.4 meter mirror and the James Webb Space Telescope has a 6.5 meter mirror. LUVOIR, not shown, will dwarf them both with a massive 15 meter mirror. Image: NASA
LUVOIR will not be cryogenically cooled like the JWST is, because it’s not primarily an Infrared observatory. LUVOIR will also be designed to be serviceable. In fact, the US Congress now requires all space telescopes to be serviceable.
“Congress has mandated that all future large space telescopes must be serviceable if practicable.” – Brad Peterson
LUVOIR is designed to have a long life. It’s multiple instruments will be replaceable, and the hope is that it will last in space for 50 years. Whether it will be serviced by robots, or by astronauts, has not been determined. It may even be designed so that it could be brought back from L2 for servicing.
LUVOIR will contribute to the search for life on other worlds. A key requirement for LUVOIR is that it do spectroscopy on the atmospheres of distant planets. If you can do spectroscopy, then you can determine habitability, and, potentially, even if a planet is inhabited. This is the first main technological challenge for LUVOIR. This spectroscopy requires a powerful coronagraph to suppress the light of the stars that exoplanets orbit. LUVOIR’s coronagraph will excel at this, with a ratio of starlight suppression of 10 billion to 1. With this capability, LUVOIR should be able to do spectroscopy on the atmospheres of small, terrestrial exoplanets, rather than just larger gas giants.
“This telescope is going to be remarkable. The key science that it’s going to do be able to do is spectroscopy of planets in the habitable zone around nearby stars.” – Brad Peterson
This video from NASA’s Goddard Space Flight Center talks about the search for life, and how telescopes like LUVOIR will contribute to the search. At the 15:00 mark, Dr. Aki Roberge talks about how spectroscopy is key to finding signs of life on exoplanets, and how LUVOIR will take that search one step further.
Using spectroscopy to search for signs of life on exoplanets is just one of LUVOIR’s science goals.
LUVOIR is tasked with other challenges as well, including:
Mapping the distribution of dark matter in the Universe.
Isolating the source of gravitational waves.
Imaging circumstellar disks to see how planets form.
Identifying the first starlight in the Universe, studying early galaxies and finding the first black holes.
Studying surface features of worlds in our Solar System.
To tackle all these challenges, LUVOIR will have to clear other technological hurdles. One of them is the requirement for long exposure times. This puts enormous constraints on the stability of the scope, since its mirror is so large. A system of active supports for the mirror segments will help with stability. This is a trait it shares with other terrestrial Super Telescopes like the Thirty Meter Telescope and the European Extremely Large Telescope. Each of those had hundreds of segments which have to be controlled precisely with computers.
A circumstellar disk of debris around a matured stellar system may indicate that Earth-like planets lie within. LUVOIR will be able to see inside the disk to watch planets forming. Credit: NASA
LUVOIR’s construction, and how it will be placed in orbit are also significant considerations.
According to Peterson, LUVOIR could be launched on either of the heavy lift rockets being developed. The Falcon Heavy is being considered, as is the Space Launch System. The SLS Block 1B could do it, depending on the final size of LUVOIR.
“I’s going to require a heavy lift vehicle.” – Brad Peterson
Or, LUVOIR may never be launched into space. It could be assembled in space with pre-built components that are launched one at a time, just like the International Space Station. There are several advantages to that.
With assembly in space, the telescope doesn’t have to be built to withstand the tremendous force it takes to launch something into orbit. It also allows for testing when completed, before being sent to L2. Once the ‘scope was assembled and tested, a small ion propulsion engine could be used to power it to L2.
It’s possible that the infrastructure to construct LUVOIR in space will exist in a decade or two. NASA’s Deep Space Gateway in cis-lunar space is planned for the mid-20s. It would act as a staging point for deep-space missions, and for missions to the lunar surface.
LUVOIR is still in the early stages. The people behind it are designing it to meet as many of the science goals as they can, all within the technological constraints of our time. Planning has to start somewhere, and the plans presented by Brad Peterson represent the current thinking behind LUVOIR. But there’s still a lot of work to do.
“Typical time scale from selection to launch of a flagship mission is something like 20 years.” – Brad Peterson
As Peterson explains, LUVOIR will have to be chosen as NASA’s highest priority during the 2020 Decadal Survey. Once that occurs, then a couple more years are required to really flesh out the design of the mission. According to Peterson, “Typical time scale from selection to launch of a flagship mission is something like 20 years.” That gets us to a potential launch in the mid-2030s.
Along the way, LUVOIR will be given a more suitable name. James Webb, Hubble, Kepler and others have all had important missions named after them. Perhaps its Carl Sagan’s turn.
“The Carl Sagan Space Telescope” has a nice ring to it, doesn’t it?
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).
Back in February of 2017, NASA announced the discovery of a seven-planet system orbiting a nearby star. This system, known as TRAPPIST-1, is of particular interest to astronomers because of the nature and orbits of the planets. Not only are all seven planets terrestrial in nature (i.e. rocky), but three of the seven have been confirmed to be within the star’s habitable zone (aka. “Goldilocks Zone”).
But beyond the chance that some of these planets could be inhabited, there is also the possibility that their proximity to each other could allow for life to be transferred between them. That is the possibility that a team of scientists from the University of Chicago sought to address in a new study. In the end, they concluded that bacteria and single-celled organisms could be hopping from planet to planet.
Artist's depiction of a waterworld. A new study suggests that Earth is in a minority when it comes to planets, and that most habitable planets may be greater than 90% ocean. Credit: David A. Aguilar (CfA)
If we want to send spacecraft to exoplanets to search for life, we better get good at building submarines.
A new study by Dr. Fergus Simpson, of the Institute of Cosmos Sciences at the University of Barcelona, shows that our assumptions about exo-planets may be wrong. We kind of assume that exoplanets will have land masses, even though we don’t know that. Dr. Simpson’s study suggests that we can expect lots of oceans on the habitable worlds that we might discover. In fact, ocean coverage of 90% may be the norm.
At the heart of this study is something called ‘Bayesian Statistics’, or ‘Bayesian Probability.’
Normally, we give something a probability of occurring—in this case a habitable world with land masses—based on our data. And we’re more confident in our prediction if we have more data. So if we find 10 exoplanets, and 7 of them have significant land masses, we think there’s a 70% chance that future exoplanets will have significant land masses. If we find 100 exoplanets, and 70 of them have significant land masses, then we’re even more confident in our 70% prediction.
Is Earth in the range of normal when it comes to habitable planets? Or is it an outlier, with both large land masses, and large oceans? Image: Reto Stöckli, Nazmi El Saleous, and Marit Jentoft-Nilsen, NASA GSFC
But the problem is, even though we’ve discovered lots of exoplanets, we don’t know if they have land masses or not. We kind of assume they will, even though the masses of those planets is lower than we expect. This is where the Bayesian methods used in this study come in. They replace evidence with logic, sort of.
In Bayesian logic, probability is assigned to something based on the state of our knowledge and on reasonable expectations. In this case, is it reasonable to expect that habitable exoplanets will have significant landmasses in the same way that Earth does? Based on our current knowledge, it isn’t a reasonable expectation.
According to Dr. Simpson, the anthropic principle comes into play here. We just assume that Earth is some kind of standard for habitable worlds. But, as the study shows, that may not be the case.
“Based on the Earth’s ocean coverage of 71%, we find substantial evidence supporting the hypothesis that anthropic selection effects are at work.” – Dr. Fergus Simpson.
In fact, Earth may be a very finely balanced planet, where the amount of water is just right for there to be significant land masses. The size of the oceanic basins is in tune with the amount of water that Earth retains over time, which produces the continents that rise above the seas. Is there any reason to assume that other worlds will be as finely balanced?
Dr. Simpson says no, there isn’t. “A scenario in which the Earth holds less water than most other habitable planets would be consistent with results from simulations, and could help explain why some planets have been found to be a bit less dense than we expected.” says Simpson.
Simpson’s statistical model shows that oceans dominate other habitable worlds, with most of them being 90% water by surface area. In fact, Earth is very close to being a water world. The video shows what would happen to Earth’s continents if the amount of water increased. There is only a very narrow window in which Earth can have both large land masses, and large oceans.
Dr. Simpson suggests that the fine balance between land and water on Earth’s surface could be one reason we evolved here. This is based partly on his model, which shows that land masses will have larger deserts the smaller the oceans are. And deserts are not the most hospitable place for life, and neither are they biodiverse. Also, biodiversity on land is about 25 times greater than biodiversity in oceans, at least on Earth.
Simpson says that the fine balance between land mass and ocean coverage on Earth could be an important reason why we are here, and not somewhere else.
“Our understanding of the development of life may be far from complete, but it is not so dire that we must adhere to the conventional approximation that all habitable planets have an equal chance of hosting intelligent life,” Simpson concludes.
Artist's impression of Kepler-1649b, the "Venus-like" world orbiting an M-class star 219 light-years from Earth. Credit: Danielle Futselaar
It has been an exciting time for exoplanet research of late! Back in February, the world was astounded when astronomers from the European Southern Observatory (ESO) announced the discovery of seven planets in the TRAPPIST-1 system, all of which were comparable in size to Earth, and three of which were found to orbit within the star’s habitable zone.
And now, a team of international astronomers has announced the discovery of an extra-solar body that is similar to another terrestrial planet in our own Solar System. It’s known as Kepler-1649b, a planet that appears to be similar in size and density to Earth and is located in a star system just 219 light-years away. But in terms of its atmosphere, this planet appears to be decidedly more “Venus-like” (i.e. insanely hot!)
Diagram comparing the Solar System to Kepler 69 and its system of exoplanets. Credit: NASA Ames/JPL-Caltech
Needless to say, this discovery is a significant one, and the implications of it go beyond exoplanet research. For some time, astronomers have wondered how – given their similar sizes, densities, and the fact that they both orbit within the Sun’s habitable zone – that Earth could develop conditions favorable to life while Venus would become so hostile. As such, having a “Venus-like” planet that is close enough to study presents some exciting opportunities.
In the past, the Kepler mission has located several extra-solar planets that were similar in some ways to Venus. For instance, a few years ago, astronomers detected a Super-Earth – Kepler-69b, which appeared to measure 2.24 times the diameter of Earth – that was in a Venus-like orbit around its host the star. And then there was GJ 1132b, a Venus-like exoplanet candidate that is about 1.5 times the mass of Earth, and located just 39 light-years away.
In addition, dozens of smaller planet candidates have been discovered that astronomers think could have atmospheres similar to that of Venus. But in the case of Kepler-1649b, the team behind the discovery were able to determine that the planet had a sub-Earth radius (similar in size to Venus) and receives a similar amount of light (aka. incident flux) from its star as Venus does from Earth.
However, they also noted that the planet also differs from Venus in a few key ways – not the least of which are its orbital period and the type of star it orbits. As Dr. Angelo told Universe Today via email:
“The planet is similar to Venus in terms of it’s size and the amount of light it receives from it’s host star. This means it could potentially have surface temperatures similar to Venus as well. It differs from Venus because it orbits a star that is much smaller, cooler, and redder than our sun. It completes its orbit in just 9 days, which places it close to its host star and subjects it to potential factors that Venus does not experience, including exposure to magnetic radiation and tidal locking. Also, since it orbits a cooler star, it receives more lower-energy radiation from its host star than Earth receives from the Sun.”
Artist’s impression of a Venus-like exoplanet orbiting close to its host star. Credit: CfA/Dana Berry
In other words, while the planet appears to receive a comparable amount of light/heat from its host star, it is also subject to far more low-energy radiation. And as a potentially tidally-locked planet, the surface’s exposure to this radiation would be entirely disproportionate. And last, its proximity to its star means it would be subject to greater tidal forces than Venus – all of which has drastic implications for the planet’s geological activity and seasonal variations.
Despite these differences, Kepler-1649b remains the most Venus-like planet discovered to date. Looking to the future, it is hoped that next-generations instruments – like the Transiting Exoplanet Survey Satellite (TESS), the James Webb Telescope and the Gaia spacecraft – will allow for more detailed studies. From these, astronomers hope to more accurately determine the size and distance of the planet, as well as the temperature of its host star.
This information will, in turn, help us learn a great deal more about what goes into making a planet “habitable”. As Angelo explained:
“Understanding how hotter planets develop thick, Venus-like atmospheres that make them inhabitable will be important in constraining our definition of a ‘habitable zone’. This may become possible in the future when we develop instruments sensitive enough to determine chemical compositions of planet atmospheres (around dim stars) using a method called ‘transit spectroscopy’, which looks at the light from the host star that has passed through the planet’s atmosphere during transit.”
The development of such instruments will be especially useful given joust how many exoplanets are being detected around neighboring red dwarf stars. Given that they account for roughly 85% of stars in the Milky Way, knowing whether or not they can have habitable planets will certainly be of interest!
A full-scale model of the JWST went on a bit of a World Tour. Here it is in Munich, Germany. Image Credit: EADS Astrium
We humans have an insatiable hunger to understand the Universe. As Carl Sagan said, “Understanding is Ecstasy.” But to understand the Universe, we need better and better ways to observe it. And that means one thing: big, huge, enormous telescopes.
In this series we’ll look at 6 of the world’s Super Telescopes:
The James Webb Space Telescope“>James Webb Space Telescope (JWST, or the Webb) may be the most eagerly anticipated of the Super Telescopes. Maybe because it has endured a tortured path on its way to being built. Or maybe because it’s different than the other Super Telescopes, what with it being 1.5 million km (1 million miles) away from Earth once it’s operating.
The JWST will do its observing while in what’s called a halo orbit at L2, a sort of gravitationally neutral point 1.5 million km from Earth. Image: NASA/JWST
If you’ve been following the drama behind the Webb, you’ll know that cost overruns almost caused it to be cancelled. That would’ve been a real shame.
The JWST has been brewing since 1996, but has suffered some bumps along the road. That road and its bumps have been discussed elsewhere, so what follows is a brief rundown.
Initial estimates for the JWST were a $1.6 billion price tag and a launch date of 2011. But the costs ballooned, and there were other problems. This caused the House of Representatives in the US to move to cancel the project in 2011. However, later that same year, US Congress reversed the cancellation. Eventually, the final cost of the Webb came to $8.8 billion, with a launch date set for October, 2018. That means the JWST’s first light will be much sooner than the other Super Telescopes.
The business end of the James Webb Space Telescope is its 18-segment primary mirror. The gleaming, gold-coated beryllium mirror has a collecting area of 25 square meters. Image: NASA/Chris Gunn
The Webb was envisioned as a successor to the Hubble Space Telescope, which has been in operation since 1990. But the Hubble is in Low Earth Orbit, and has a primary mirror of 2.4 meters. The JWST will be located in orbit at the LaGrange 2 point, and its primary mirror will be 6.5 meters. The Hubble observes in the near ultraviolet, visible, and near infrared spectra, while the Webb will observe in long-wavelength (orange-red) visible light, through near-infrared to the mid-infrared. This has some important implications for the science yielded by the Webb.
The Webb’s Instruments
The James Webb is built around four instruments:
The Near-Infrared Camera (NIRCam)
The Near-Infrared Spectrograph (NIRSpec)
The Mid-Infrared Instrument(MIRI)
The Fine Guidance Sensor/ Near InfraRed Imager and Slitless Spectrograph (FGS/NIRISS)
This image shows the wavelengths of the infrared spectrum that Webb’s instruments can observe. Image: NASA/JWST
The NIRCam is Webb’s primary imager. It will observe the formation of the earliest stars and galaxies, the population of stars in nearby galaxies, Kuiper Belt Objects, and young stars in the Milky Way. NIRCam is equipped with coronagraphs, which block out the light from bright objects in order to observe dimmer objects nearby.
NIRSpec will operate in a range from 0 to 5 microns. Its spectrograph will split the light into a spectrum. The resulting spectrum tells us about an objects, temperature, mass, and chemical composition. NIRSpec will observe 100 objects at once.
MIRI is a camera and a spectrograph. It will see the redshifted light of distant galaxies, newly forming stars, objects in the Kuiper Belt, and faint comets. MIRI’s camera will provide wide-field, broadband imaging that will rank up there with the astonishing images that Hubble has given us a steady diet of. The spectrograph will provide physical details of the distant objects it will observe.
The Fine Guidance Sensor part of FGS/NIRISS will give the Webb the precision required to yield high-quality images. NIRISS is a specialized instrument operating in three modes. It will investigate first light detection, exoplanet detection and characterization, and exoplanet transit spectroscopy.
The Science
The over-arching goal of the JWST, along with many other telescopes, is to understand the Universe and our origins. The Webb will investigate four broad themes:
First Light and Re-ionization: In the early stages of the Universe, there was no light. The Universe was opaque. Eventually, as it cooled, photons were able to travel more freely. Then, probably hundreds of millions of years after the Big Bang, the first light sources formed: stars. But we don’t know when, or what types of stars.
How Galaxies Assemble: We’re accustomed to seeing stunning images of the grand spiral galaxies that exist in the Universe today. But galaxies weren’t always like that. Early galaxies were often small and clumpy. How did they form into the shapes we see today?
The Birth of Stars and Protoplanetary Systems: The Webb’s keen eye will peer straight through clouds of dust that ‘scopes like the Hubble can’t see through. Those clouds of dust are where stars are forming, and their protoplanetary systems. What we see there will tell us a lot about the formation of our own Solar System, as well as shedding light on many other questions.
Planets and the Origins of Life: We now know that exoplanets are common. We’ve found thousands of them orbiting all types of stars. But we still know very little about them, like how common atmospheres are, and if the building blocks of life are common.
These are all obviously fascinating topics. But in our current times, one of them stands out among the others: Planets and the Origins of Life.
The recent discovery the TRAPPIST 1 system has people excited about possibly discovering life in another solar system. TRAPPIST 1 has 7 terrestrial planets, and 3 of them are in the habitable zone. It was huge news in February 2017. The buzz is still palpable, and people are eagerly awaiting more news about the system. That’s where the JWST comes in.
One big question around the TRAPPIST system is “Do the planets have atmospheres?” The Webb can help us answer this.
The NIRSpec instrument on JWST will be able to detect any atmospheres around the planets. Maybe more importantly, it will be able to investigate the atmospheres, and tell us about their composition. We will know if the atmospheres, if they exist, contain greenhouse gases. The Webb may also detect chemicals like ozone and methane, which are biosignatures and can tell us if life might be present on those planets.
You could say that if the James Webb were able to detect atmospheres on the TRAPPIST 1 planets, and confirm the existence of biosignature chemicals there, it will have done its job already. Even if it stopped working after that. That’s probably far-fetched. But still, the possibility is there.
Launch and Deployment
The science that the JWST will provide is extremely intriguing. But we’re not there yet. There’s still the matter of JWST’s launch, and it’s tricky deployment.
The JWST’s primary mirror is much larger than the Hubble’s. It’s 6.5 meters in diameter, versus 2.4 meters for the Hubble. The Hubble was no problem launching, despite being as large as a school bus. It was placed inside a space shuttle, and deployed by the Canadarm in low earth orbit. That won’t work for the James Webb.
This image shows the Hubble Space Telescope being held above the shuttle’s cargo bay by the Canadian-built Remote Manipulator System (RMS) arm, or Canadarm. A complex operation, but not as complex as JWST’s deployment. Image: NASA
The Webb has to be launched aboard a rocket to be sent on its way to L2, it’s eventual home. And in order to be launched aboard its rocket, it has to fit into a cargo space in the rocket’s nose. That means it has to be folded up.
The mirror, which is made up of 18 segments, is folded into three inside the rocket, and unfolded on its way to L2. The antennae and the solar cells also need to unfold.
Unlike the Hubble, the Webb needs to be kept extremely cool to do its work. It has a cryo-cooler to help with that, but it also has an enormous sunshade. This sunshade is five layers, and very large.
We need all of these components to deploy for the Webb to do its thing. And nothing like this has been tried before.
The Webb’s launch is only 7 months away. That’s really close, considering the project almost got cancelled. There’s a cornucopia of science to be done once it’s working.
But we’re not there yet, and we’ll have to go through the nerve-wracking launch and deployment before we can really get excited.
Image of the Sarychev volcano (in Russia's Kuril Islands) caught during an early stage of eruption on June 12, 2009. Taken by astronauts aboard the
International Space Station. Credit: NASA
Whenever the existence of an extra-solar planet is confirmed, there is reason to celebrate. With every new discovery, humanity increases the odds of finding life somewhere else in the Universe. And even if that life is not advanced enough (or particularly inclined) to build a radio antenna so we might be able to hear from them, even the possibility of life beyond our Solar System is exciting.
Unfortunately, determining whether or not a planet is habitable is difficult and subject to a lot of guesswork. While astronomers use various techniques to put constraints on the size, mass, and composition of extra-solar planets, there is no surefire way to know if these worlds are habitable. But according to a new study from a team of astronomers from Cornell University, looking for signs of volcanic activity could help.
Their study – titled “A Volcanic Hydrogen Habitable Zone” – was recently published in The Astrophysical Journal Letters. According to their findings, the key to zeroing in on life on other planets is to look for the telltale signs of volcanic eruptions – namely, hydrogen gas (H²). The reason being is that this, and the traditional greenhouse gases, could extend the habitable zones of stars considerably.
The habitable zones of three stars detected by the Kepler mission. Credit: NASA/Ames/JPL-Caltech
As Ramses Ramirez, a research associate at Cornell’s Carl Sagan Institute and the lead author of the study, said in a University press release:
“On frozen planets, any potential life would be buried under layers of ice, which would make it really hard to spot with telescopes. But if the surface is warm enough – thanks to volcanic hydrogen and atmospheric warming – you could have life on the surface, generating a slew of detectable signatures.”
Planetary scientists theorize that billions of years ago, Earth’s early atmosphere had an abundant supply of hydrogen gas (H²) due to volcanic outgassing. Interaction between hydrogen and nitrogen molecules in this atmosphere are believed to have kept the Earth warm long enough for life to develop. However, over the next few million years, this hydrogen gas escaped into space.
This is believed to be the fate of all terrestrial planets, which can only hold onto their planet-warming hydrogen for so long. But according to the new study, volcanic activity could change this. As long as they are active, and their activity is intense enough, even planets that are far from their stars could experience a greenhouse effect that would be sufficient to keep their surfaces warm.
Distant exoplanets that are not in the traditional “Goldilocks Zone” might be habitable, assuming they have enough volcanic activity. Credit: ESO.
Consider the Solar System. When accounting for the traditional greenhouse effect caused by nitrogen gas (N²), carbon dioxide and water, the outer edge of our Sun’s habitable zone extends to a distance of about 1.7 AU – just outside the orbit of Mars. Beyond this, the condensation and scattering of CO² molecules make a greenhouse effect negligible.
However, if one factors in the outgassing of sufficient levels of H², that habitable zone can extend that outer edge to about 2.4 AUs. At this distance, planets that are the same distance from the Sun as the Asteroid Belt would theoretically be able to sustain life – provided enough volcanic activity was present. This is certainly exciting news, especially in light of the recent announcement of seven exoplanets orbiting the nearby TRAPPIST-1 star.
Of these planets, three are believed to orbit within the star’s habitable zone. But as Lisa Kaltenegger – also a member of the Carl Sagan Institute and the co-author on the paper – indicated, their research could add another planet to this
“potentially-habitable” lineup:
“Finding multiple planets in the habitable zone of their host star is a great discovery because it means that there can be even more potentially habitable planets per star than we thought. Finding more rocky planets in the habitable zone – per star – increases our odds of finding life… Although uncertainties with the orbit of the outermost Trappist-1 planet ‘h’ means that we’ll have to wait and see on that one.”
Artist’s concept of the TRAPPIST-1 star system, an ultra-cool dwarf that has seven Earth-size planets orbiting it. Credits: NASA/JPL-Caltech
Another upside of this study is that the presence of volcanically-produced hydrogen gas would be easy to detect by both ground-based and space-based telescopes (which routinely conduct spectroscopic surveys on distant exoplanets). So not only would volcanic activity increase the likelihood of there being life on a planet, it would also be relatively easy to confirm.
“We just increased the width of the habitable zone by about half, adding a lot more planets to our ‘search here’ target list,” said Ramirez. “Adding hydrogen to the air of an exoplanet is a good thing if you’re an astronomer trying to observe potential life from a telescope or a space mission. It increases your signal, making it easier to spot the makeup of the atmosphere as compared to planets without hydrogen.”
Already, missions like Spitzer and the Hubble Space Telescope are used to study exoplanets for signs of hydrogen and helium – mainly to determine if they are gas giants or rocky planets. But by looking for hydrogen gas along with other biosignatures (i.e. methane and ozone), next-generation instruments like the James Webb Space Telescope or the European Extremely Large Telescope, could narrow the search for life.
It is, of course, far too soon to say if this study will help in our search for extra-solar life. But in the coming years, we may find ourselves one step closer to resolving that troublesome Fermi Paradox!
Artist's concept of the TRAPPIST-1 star system, an ultra-cool dwarf that has seven Earth-size planets orbiting it. We're going to keep finding more and more solar systemsl like this, but we need observatories like WFIRST, with starshades, to understand the planets better. Credits: NASA/JPL-Caltech
NASA’s announcement last week of 7 new exoplanets is still causing great excitement. Any time you discover 7 “Earth-like” planets around a distant star, with 3 of them “potentially” in the habitable zone, it’s a big deal. But now that we’re over some of our initial excitement, let’s look at some of the questions that need to be answered before we can all get excited again.
What About That Star?
The star that the planets orbit, called Trappist-1, is a Red Dwarf star, much dimmer and cooler than our Sun. The three potentially habitable planets—TRAPPIST-1e, f, and g— get about the same amount of energy as Earth and Mars do from the Sun, because they’re so close to it. Red Dwarfs are very long-lasting stars, and their lifetimes are measured in the trillions of years, rather than billions of years, like our Sun is.
But Red Dwarfs themselves can have some unusual properties that are problematic when it comes to supporting life on nearby planets.
This illustration shows TRAPPIST-1 in relation to our Sun. Image: By ESO – http://www.eso.org/public/images/eso1615e/, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=48532941
Red Dwarfs can be covered in starspots, or what we call sunspots when they appear on our Sun. On our Sun, they don’t have much affect on the amount of energy received by the Earth. But on a Red Dwarf, they can reduce the energy output by up to 40%. And this can go on for months at a time.
Other Red Dwarfs can emit powerful flares of energy, causing the star to double in brightness in mere minutes. Some Red Dwarfs constantly emit these flares, along with powerful magnetic fields.
Part of the excitement surrounding the Trappist planets is that they show multiple rocky planets in orbit around a Red Dwarf. And Red Dwarfs are the most common type of star in the Milky Way. So, the potential for life-supporting, rocky planets just grew in a huge way.
But we don’t know yet how the starspots and flaring of Red Dwarfs will affect the potential habitability of planets orbiting them. It could very well render them uninhabitable.
Will Tidal Locking Affect the Planets’ Habitability?
The planets orbiting Trappist-1 are very likely tidally locked to their star. This means that they don’t rotate, like Earth and the rest of the planets in our Solar System. This has huge implications for the potential habitability of these planets. With one side of the planet getting all the energy from the star, and the other side in perpetual darkness, these planets would be nothing like Earth.
Tidal locking is not rare. For example, Pluto and its moon Charon (above) are tidally locked to each other, as are the Earth and the Moon. But can life appear and survive on a planet tidally locked to its star? Credit: NASA/JHUAPL/SwRI
One side would be constantly roasted by the star, while the other would be frigid. It’s possible that some of these planets could have atmospheres. Depending on the type of atmosphere, the extreme temperature effects of tidal locking could be mitigated. But we just don’t know if or what type of atmosphere any of the planets have. Yet.
So, Do They Have Atmospheres?
We just don’t know yet. But we do have some constraints on what any atmospheres might be.
Preliminary data from the Hubble Space Telescope suggests that TRAPPIST 1b and 1c don’t have extended gas envelopes. All that really tells us is that they aren’t gaseous planets. In any case, those two planets are outside of the habitable zone. What we really need to know is if TRAPPIST 1e, 1f, and 1g have atmospheres. We also need to know if they have greenhouse gases in their atmospheres. Greenhouse gases could help make tidally locked planets hospitable to life.
On a tidally locked planet, the termination line between the sunlit side and the dark side is considered the most likely place for life to develop. The presence of greenhouse gases could expand the habitable band of the termination line and make more of the dark side warmer.
We won’t know much about any greenhouse gases in the atmospheres of these planets until the James Webb Space Telescope (JWST) and the European Extremely Large Telescope (EELT) are operating. Those two ‘scopes will be able to analyze the atmospheres for greenhouse gases. They might also be able to detect biosignatures like ozone and methane in the atmospheres.
We’ll have to wait a while for that though. The JWST doesn’t launch until October 2018, and the EELT won’t see first light until 2024.
Do They Have Liquid Water?
We don’t know for sure if life requires liquid water. We only know that’s true on Earth. Until we find life somewhere else, we have to be guided by what we know of life on Earth. So we always start with liquid water.
A study published in 2016 looked at planets orbiting ultra-cool dwarfs like TRAPPIST-1. They determined that TRAPPIST 1b and 1c could have lost as much as 15 Earth oceans of water during the early hot phase of their solar system. TRAPPIST 1d might have lost as much as 1 Earth ocean of water. If they had any water initially, that is. But the study also shows that they may have retained some of that water. It’s not clear if the three habitable planets in the TRAPPIST system suffered the same loss of initial water. But if they did, they could have retained a similar amount of water.
Artist’s impression of an “eyeball” planet, a water world where the sun-facing side is able to maintain a liquid-water ocean. Credit and Copyright: eburacum45/ DeviantArt
There are still a lot of questions here. The word “habitable” only means that they are receiving enough energy from their star to keep water in liquid form. Since the planets are tidally locked, any water they did retain could be frozen on the planets’ dark side. To find out for sure, we’ll have to point other instruments at them.
Are Their Orbits Stable?
Planets require stable orbits over a biologically significant period of time in order for life to develop. Conditions that change too rapidly make it impossible for life to survive and adapt. A planet needs a stable amount of solar radiation, and a stable temperature, to support life. If the solar radiation, and the planet’s temperature, fluctuates too rapidly or too much due to orbital instability, then life would not be able to adapt to those changes.
Right now, there’s no indication that the orbits of the TRAPPIST 1 planets are unstable. But we are still in the preliminary stage of investigation. We need a longer sampling of their orbits to know for sure.
Pelted by Interlopers?
Our Solar System is a relatively placid place when it comes to meteors and asteroids. But it wasn’t always that way. Evidence from lunar rock samples show that it may have suffered through a period called the “Late Heavy Bombardment.” During this time, the inner Solar System was like a shooting gallery, with Earth, Venus, Mercury, Mars, and our Moon being struck continuously by asteroids.
The cause of this period of Bombardment, so the theory goes, was the migration of the giant planets through the solar system. Their gravity would have dislodged asteroids from the asteroid belt and the Kuiper Belt, and sent them into the path of the inner, terrestrial planets.
We know that Earth has been hit by meteorites multiple times, and that at least one of those times, a mass extinction was the result.
Computer generated simulation of an asteroid strike on the Earth. Credit: Don Davis/AFP/Getty Images
The TRAPPIST 1 system has no giant planets. But we don’t know if it has an asteroid belt, a Kuiper Belt, or any other organized, stable body of asteroids. It may be populated by asteroids and comets that are unstable. Perhaps the planets in the habitable zone are subjected to regular asteroid strikes which wipes out any life that gets started there. Admittedly, this is purely speculative, but so are a lot of other things about the TRAPPIST 1 system.
How Will We Find Out More?
We need more powerful telescopes to probe exoplanets like those in the TRAPPIST 1 system. It’s the only way to learn more about them. Sending some kind of probe to a solar system 40 light years away is something that might not happen for generations, if ever.
Luckily, more powerful telescopes are on the way. The James Webb Space Telescope should be in operation by April of 2019, and one of its objectives is to study exoplanets. It will tell us a lot more about the atmospheres of distant exoplanets, and whether or not they can support life.
Other telescopes, like the Giant Magellan Telescope (GMT) and the European Extremely Large Telescope (E-ELT), have the potential to capture images of large exoplanets, and possibly even Earth-sized exoplanets like the ones in the TRAPPIST system. These telescopes will see their first light within ten years.
This artist’s impression shows the European Extremely Large Telescope (E-ELT) in its enclosure. The E-ELT will be a 39-metre aperture optical and infrared telescope. ESO/L. Calçada
What these questions show is that we can’t get ahead of ourselves. Yes, it’s exciting that the TRAPPIST planets have been discovered. It’s exciting that there are multiple terrestrial worlds there, and that 3 of them appear to be in the habitable zone.
It’s exciting that a Red Dwarf star—the most common type of star in our neighborhood—has been found with multiple rocky planets in the habitable zone. Maybe we’ll find a bunch more of them, and the prospect of finding life somewhere else will grow.
But it’s also possible that Earth, with all of its life supporting and sustaining characteristics, is an extremely unlikely occurrence. Special, rare, and unrepeatable.
This artist's illustration shows planetisimals around a young star. New research shows that planetesimals are blasted by headwind, losing debris into space. Image Credit: NASA/JPL
The theory of how planets form has been something of an enduring mystery for scientists. While astronomers have a pretty good understanding of where planetary systems comes from – i.e. protoplanetary disks of dust and gas around new stars (aka. “Nebular Theory“) – a complete understanding of how these discs eventually become objects large enough to collapse under their own gravity has remained elusive.
But thanks to a new study by a team of researchers from France, Australia and the UK, it seems that the missing piece of the puzzle may finally have been found. Using a series of simulations, these researchers have shown how “dust traps” – i.e. regions where pebble-sized fragments could collect and stick together – are common enough to allow for the formation of planetesimals.
An image of a protoplanetary disk, made using results from the new model, after the formation of a spontaneous dust trap, visible as a bright dust ring. Gas is depicted in blue and dust in red. Credit: Jean-Francois Gonzalez.
Until recently, the process by which protoplanetary disks of dust and gas aggregate to form peddle-sized objects, and the process by which planetesimals (objects that are one hundred meters or more in diameter) form planetary cores, have been well understood. But the process that bridges these two – where pebbles come together to form planetesimals – has remained unknown.
Part of the problem has been the fact that the Solar System, which has been our only frame of reference for centuries, formed billions of years ago. But thanks to recent discoveries (3453 confirmed exoplanets and counting), astronomers have had lots of opportunities to study other systems that are in various stages of formation. As Dr. Gonzalez explained in a Royal Astronomical Society press release:
“Until now we have struggled to explain how pebbles can come together to form planets, and yet we’ve now discovered huge numbers of planets in orbit around other stars. That set us thinking about how to solve this mystery.”
In the past, astronomers believed that “dust traps” – which are integral to planet formation – could only exist within certain environments. In these high-pressure regions, large grains of dust are slowed down to the point where they are able to come together. These regions are extremely important since they counteract the two main obstacles to planetary formation, which are drag and high-speed collisions.
Artist’s impression of the planets in our solar system, along with the Sun (at bottom). Credit: NASA
Drag is caused by the effect gas has on dust grains, which causes them to slow down and eventually drift into the central star (where they are consumed). As for high-speed collisions, this is what causes large pebbles to smash into each other and break apart, thus reversing the aggregation process. Dust traps are therefore needed to ensure that dust grains are slowed down just enough so that they won’t annihilate each other when they collide.
To see just how common these dust traps were, Dr. Gonzalez and his colleagues conducted a series of computer simulations that took into account how dust in a protoplanetary disk could exert drag on the gas component – a process known as “aerodynamic drag back-reaction”. Whereas gas typically has an arresting influence on dust particles, in particularly dusty rings, the opposite can be true.
This effect has been largely ignored by astronomers up until recently, since its generally quite negligible. But as the team noted, it is an important factor in protoplanetary disks, which are known for being incredibly dusty environments. In this scenario, the effect of back-reaction is to slow inward-moving dust grains and push gas outwards where it forms high-pressure regions – i.e. “dust traps”.
Once they accounted for these effects, their simulations showed how planets form in three basic stages. In the first stage, dust grains grow in size and move inwards towards the central star. In the second, the now pebble-sized larger grains accumulate and slow down. In the third and final stage, the gas is pushed outwards by the back-reaction, creating the dust trap regions where it accumulates.
These traps then allow the pebbles to aggregate to form planetesimals, and eventually planet-sized worlds. With this model, astronomers now have a solid idea of how planetary formation goes from dusty disks to planetesimals coming together. In addition to resolving a key question as to how the Solar System came to be, this sort of research could prove vital in the study of exoplanets.
Ground-based and space-based observatories have already noted the presence of dark and bright rings that are forming in protoplanetary disks around distant stars – which are believed to be dust traps. These systems could provide astronomers with a chance to test this new model, as they watch planets slowly come together. As Dr. Gonzalez indicated:
“We were thrilled to discover that, with the right ingredients in place, dust traps can form spontaneously, in a wide range of environments. This is a simple and robust solution to a long standing problem in planet formation.”
This artist's concept shows what each of the TRAPPIST-1 planets may look like, based on available data about their sizes, masses and orbital distances.
Credits: NASA/JPL-Caltech
The Trappist-1 system has been featured in the news quite a bit lately. In May of 2016, it appeared in the headlines after researchers announced the discovery of three exoplanets orbiting around the red dwarf star. And then there was the news earlier this week of how follow-up examinations from ground-based telescopes and the Spitzer Space Telescope revealed that there were actually seven planets in this system.
And now it seems that there is more news to be had from this star system. As it turns out, the Search for Extraterrestrial Intelligence (SETI) Institute was already monitoring this system with their Allen Telescope Array (ATA), looking for signs of life even before the multi-planet system was announced. And while the survey did not detect any telltale signs of radio traffic, further surveys are expected.
Given its proximity to our own Solar System, and the fact that this system contains seven planets that are similar in size and mass to Earth, it is both tempting and plausible to think that life could be flourishing in the TRAPPIST-1 system. As Seth Shostak, a Senior Astronomer at SETI, explained:
“[T]he opportunities for life in the Trappist 1 system make our own solar system look fourth-rate. And if even a single planet eventually produced technically competent beings, that species could quickly disperse its kind to all the rest… Typical travel time between worlds in the Trappist 1 system, even assuming rockets no speedier than those built by NASA, would be pleasantly short. Our best spacecraft could take you to Mars in 6 months. To shuttle between neighboring Trappist planets would be a weekend junket.”
Illustration showing the possible surface of TRAPPIST-1f, one of the newly discovered planets in the TRAPPIST-1 system. Credits: NASA/JPL-Caltech
Little wonder then why SETI has been using their Allen Telescope Array to monitor the system ever since exoplanets were first announced there. Located at the Hat Creek Radio Observatory in northern California (northeast of San Francisco), the ATA is what is known as a “Large Number of Small Dishes” (LNSD) array – which is a new trend in radio astronomy.
Like other LNSD arrays – such as the proposed Square Kilometer Array currently being built in Australia and South Africa – the concept calls for the deployment of many smaller dishes over a large surface area, rather than a single large dish. Plans for the array began back in 1997, when the SETI Institute convened a workshop to discuss the future of the Institute and its search strategies.
The final report of the workshop, titled “SETI 2020“, laid out a plan for the creation of a new telescope array. This array was referred to as the One Hectare Telescope at the time, since the plan called for a LNSD encompassing an area measuring 10,000 m² (one hectare). The SETI Institute began developing the project in conjunction with the Radio Astronomy Laboratory (RAL) at the UC Berkeley.
In 2001, they secured a $11.5 million donation from the Paul G. Allen Family Foundation, which was established by Microsoft co-founder Paul Allen. In 2007, the first phase of construction was completed and the ATA finally became operational on October 11th, 2007, with 42 antennas (ATA-42). Since that time, Allen has committed to an additional $13.5 million in funding for a second phase of expansion (hence why it bears his name).
A portion of the Allen Telescope Array. (Credit: Seth Shostak/The SETI Institute. Used with permission)
Compared to large, single dish-arrays, smaller dish-arrays are more cost-effective because they can be upgraded simply by adding more dishes. The ATA is also less expensive since it relies on commercial technology originally developed for the television market, as well as receiver and cryogenic technologies developed for radio communication and cell phones.
It also uses programmable chips and software for signal processing, which allows for rapid integration whenever new technology becomes available. As such, the array is well suited to running simultaneous surveys at centimeter wavelengths. As of 2016, the SETI Institute has performed observations with the ATA for 12 hour periods (from 6 pm and 6 am), seven days a week.
And last year, the array was aimed towards TRAPPIST-1, where it conducted a survey scanning ten billion radio channels in search of signals. Naturally, the idea that a radio signal would be emanating from this system, and one which the ATA could pick up, might seem like a bit of a longshot. But in fact, both the infrastructure and energy requirements would not be beyond a species who’s technical advancement is commensurate with our own.
“Assuming that the putative inhabitants of this solar system can use a transmitting antenna as large as the 500 meter FAST radio telescope in China to beam their messages our way, then the Allen Array could have found a signal if the aliens use a transmitter with 100 kilowatts of power or more,” said Shostak. “This is only about ten times as energetic as the radar down at your local airport.”
A plot of diameter versus the amount of sunlight hitting the planets in the TRAPPIST-1 system, scaled by the size of the Earth and the amount of sunlight hitting the Earth. Credit: F. Marchis/H. Marchis
So far, nothing has been picked up from this crowded system. But the SETI Institute is not finished and future surveys are already in the works. If there is a thriving, technologically-advanced civilization in this system (and they know their way around a radio antenna), surely there will be signs soon enough.
And regardless, the discovery of seven planets in the TRAPPIST-1 system is very exciting because it demonstrates just how plentiful systems that could support life are in our Universe. Not only does this system have three planets orbiting within its habitable zone (all of which are similar in size and mass to Earth), but the fact that they orbit a red dwarf star is very encouraging.
These stars are the most common in our Universe, making up 70% of stars in our galaxy, and up to 90% in elliptical galaxies. They are also very stable, remaining in their Main Sequence phase for up to 10 trillion years. Last, but not least, astronomers believe that 20 out of 30 nearest stars to our Solar System are red dwarfs. Lots of opportunities to find life within a few dozen light years!
“[W]hether or not Trappist 1 has inhabitants, its discovery has underlined the growing conviction that the Universe is replete with real estate on which biology could both arise and flourish,’ says Shostak. “If you still think the rest of the universe is sterile, you are surely singular, and probably wrong.”
