Atmospheres of Super Earths
Written by Fraser Cain
We stand on the edge of the next phase of planetary discovery. Hundreds of massive, Jupiter-like planets have been discovered, but now astronomers are turning up smaller, more familiar planets. Planets the mass of Earth are out of reach today, but a new class of super Earth planets are now being discovered, and more will be turned up with the next generation of ground and space-based telescopes. Perhaps the most interesting research will be in the atmospheres of these planets.
Super Earths can have up to 10 times the mass of the Earth, but with a solid surface and liquid water they could very well be habitable. A recent presentation by Eliza Miller-Ricci from Harvard University at the 213th meeting of the American Astronomical Society discussed the kinds of atmospheres astronomers might see as these super Earths start turning up. Although interesting scientifically – geologic outgassing, evidence of plate tectonics, and the thickness or thinness of the atmosphere, the most interesting question will be: can super Earth planets support life?
To have life as we understand it, super Earth planets (like regular Earth planets) will need to have liquid water on their surface, and the requires a certain temperature range – the parent star's habitable zone. As we see in our own Solar System, the atmosphere of a planet helps regulate its temperature; Venus has a thick atmosphere and it's hot enough to melt lead, while Earth has a nice temperature to allow liquid water to form on its surface. Mars has a thin atmosphere and it's really cold. It's not just the thickness of the atmosphere that matters, it's also what's in it: carbon dioxide, water, etc.
High mass planets like Jupiter are mostly formed from hydrogen. Low mass terrestrial planets like Earth can't hold onto their hydrogen and it escapes into space during the planet's early history. But these super Earths might be able to hold onto their hydrogen. Instead of a low-hydrogen atmosphere like Earth, they might have an atmosphere with large quantities of water. And water is a powerful greenhouse gas – trace amounts of water vapor in Earth's atmosphere account for 60% of our greenhouse effect, keeping the planet warm and habitable.
I asked Miller-Ricci about what impact large quantities of hydrogen will have on the atmosphere of a super Earth planet. We have water here on Earth, but very little in the atmosphere. Water vapor is a powerful greenhouse gas and would help define the temperature of the planet. "The amount of hydrogen in the atmosphere of a super Earth planet would significantly affect its habitable zone. This is a really important question, it's what we're looking at next."
Current missions can detect super Earths using the transit method, where the planet dims light from its parent star as it passes in front. By subtracting the chemical signature when the planet passes behind the star, astronomers can determine its atmosphere.
Finding super Earths is at the limit of current telescopes, but more powerful instruments are launching soon. NASA's Kepler mission, launching in April 2009, will turn up even more super Earths than have already been found. But the next generation of space telescopes, like NASA's James Webb Space Telescope will allow astronomers to image these planet's atmospheres directly.
Filed under: Extrasolar Planets
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January 7th, 2009 at 10:48 pm
Presumably life as we know it would not be possible on a "super Earht" due to the crushing effect of gravity?
January 8th, 2009 at 12:13 am
so far i have not read of a single super earth discovered that lies in the habitable zone. speaking of which, in addition to the planet features, the star itself must be of the correct type. low x-rays, stable, etc…
there are so many factors to account for. even the near area to the respective solar systems must meet certain requirements.
January 8th, 2009 at 12:37 am
One fast, easy way to detect land-life resembling Earth's on any world is the presence of the products of combustion — ordinary burning, CO2, CO, soot, etc. — in its atmosphere. Combustion requires a certain percentage of atmospheric oxygen and fuel. The fuel is generally in the form of plant tissues or such compounds as petroleum, which are the products of geological processing of the corpses of plants and animals over millions, tens of millions, or hundreds of millions of years. The large quantities of atmospheric oxygen are the products of life on a world — only life, probably plant life, could continuously pump out oxygen in huge quantities long enough to build and sustain a richly oxygenated atmosphere, which is the only kind that will sustain burning. And the fuel must be on land — it won't burn in water. So products of combustion in the atmosphere of a world signal the presence on that world of a well-established land biota, if only plants. If we can observe such byproducts of combustion in the atmosphere of a super-Earth, you can bet it not only is blessed with life, but land-life, as well — and win good money at it.
January 8th, 2009 at 9:14 am
Micky, the gravity would probably not be a problem for life – microscopic life such as bacteria would almost not notice the gravity at all, and if there are larger life forms on such a planet, they would be adapted to the high gravity.
January 8th, 2009 at 11:16 am
I thought detection of super earths was possible due in part to increased diameter, not just mass. Is this true?
If the diameter of the planet is larger, than even its higher gravity/mass would probably not be that different from what we have here on earth, right?
To put it another way, IIRC, gravity diminishes with the square of distance from the center of mass. On a planet of about the same density of earth, but 10x the mass, a person standing on the surface would be considerably further from the center of gravity than one on earth would be. Likewise, for such a planet, I would imagine the amount of hydrogen retained at the surface would not be unlike what is retained here on earth (i.e. none). (Although perhaps a possible source of hydrogen replenishment exists from lower in the mantle/core of such a planet?)
Am I missing something here? (I mean apart from the fact that a planet 10x earth's mass is probably at least *somewhat* more dense due to gravity-induced pressure.)
January 8th, 2009 at 11:52 am
Mickey & Garrett
If two planets of different mass are the same density, it turns out the surface gravity is proportional to the cube root of the mass.
The cube root of 10 is about 2.15. Since pressure in the center of the planet would compress the matter to a somewhat higher density the surface gravity of a 10 earth mass planet might get as high as 3 or 4 times the gravity of earth. This would be uncomfortable for humans but should be reasonable for animals adapted to it.
January 8th, 2009 at 11:55 am
Excellent website for extrasolar planets is at
http://exoplanet.eu/catalog.php
A lot of info, catalogue of all dicscovered planets (334 as of today), description of methods, and much more. Regularly updated after each new discovery.
January 8th, 2009 at 3:45 pm
"Presumably life as we know it would not be possible on a "super Earht" due to the crushing effect of gravity?"
Why? If there are water oceans, you can still have all kinds of neutrally bouyant life in the sea.
Any land animals will not be large, but will be thick-legged and slow. No tall plants, either.
You can still have life, just don't expect anything built like giraffes or redwoods…
And as noted above, density will be an issue, too.
Intelligent life would have the devil's own time developing space flight, though.
January 8th, 2009 at 8:33 pm
The zone of habitability is VERY narrow!
The liklihood of a reasonable mass planet being in that zone is not very high.
We live on a privileged, created planet.
Cheers!
(Salacious B. Crumb, I don't want to see a scientific paper in response to my post, please.)
January 9th, 2009 at 1:18 pm
ElfOfGod Says:
" (Salacious B. Crumb, I don't want to see a scientific paper in response to my post, please.) "
But I would really love to…
ElfOfGod Says:
"The zone of habitability is VERY narrow!"
There are too many parameters and all kinds of other variables like kind of parent star, planet, its density, chemical composition, orbital inclination, orbit characteristic, type of atmosphere to dare say that zone of habitability is very narrow.
You forgot also about the fact that globe the size of super earth might be a moon orbiting the gas giant and the thermal conditions on the surface of the body may allow the water to exist in liquid state.
Probably then zone of habitability might be not only singular but also very different.
ElfOfGod Says:
"We live on a privileged, created planet."
The problem is our human way of thinking – everybody is always more prone to say that our world is the most unique.
That problem was solved quite a while ago by my fellow countryman Nicolaus Copernicus when he discovered that Earth is not the center of the universe
But well… some people have guts enough to say that the Earth is flat. Really
January 9th, 2009 at 4:48 pm
With our limited technology, and limited time that we have been looking, we already have found over 300 planets! Given better telescopes, more expertise, and more time, we will find dozens of habitable zone earth-like planets in the next 15 years! I'm so excited to see how hot the hunt will get when we think we have spotted even one! Everyone will want to get the best data/images! Let's get the robot probes primed!!!
January 10th, 2009 at 3:46 pm
Hi All
A solid super-Earth has a radius that scales to the 0.26-0.27 power of its mass, relative to Earth. Thus surface gravity scales according to the 0.48-0.46 power of the mass – a 10 Earth mass planet has about 2.88 times Earth's gravity. Adding hydrogen as water increases the radius, but the index is roughly the same. For example if the planet was 50% water – thus 5.6% hydrogen by mass – then the relative radius is roughly 1.26 times what an Earth composition planet would be. For a 10 Earth mass planet the gravity is now just 1.8 gee.
Sounds alright – you'd be heavy, but not immobilised. Exoskeletons could be strong enough to compensate. Only hassle would be that the planet is now covered in water and deeper than about 100 km it's in the form of high-pressure Ice VII. Floating would be your main activity.
January 11th, 2009 at 5:33 am
Most big planets will probably have moons around them that might have the good conditions to have life.
Life as we know here, has vey narrow conditions, but alien life could exist ot places that even humans might not exist.
January 11th, 2009 at 8:54 pm
I would not want to meet muscles that evolved in five gees. I suspect they will be little guys but strong!
January 15th, 2009 at 6:33 am
Somebody explain, why we have such a diversity in size……. We started with giant dinosaurs and have ended up with Pygmy Midgets. I think somebody is making some big presumptions about these exoplanets and basing everything on Jupiter and Saturn planets that are not even habitable!