Atmospheres of Super Earths

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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.