Mass Effect: What Exoplanet Atmospheres Can Tell Us

by Paul Gilster on December 24, 2013

Let me offer best wishes for the holidays to all Centauri Dreams readers, with thanks for the numerous comments and suggestions over the course of the past year. The schedule this week is abbreviated but I’ll have a new post up on Friday. I’m about to set out to gather the materials I need for a family dinner tonight, but I have time this morning to talk about interesting new work on figuring out the mass of an exoplanet. As you might guess, this is a key measurement, and a tough one to make. The work out of MIT offers an elegant solution.

I yield to no one in my admiration for the tough-minded Sara Seager (MIT), whose career in astrophysics is so movingly described in Lee Billings Five Billion Years of Solitude: The Search for Life Among the Stars. A number of readers pointed out Seager’s most recent study, which develops this new technique for our exoplanet toolkit. Julien de Wit, a MIT grad student who is lead author on the paper just published in Science, describes his work with Seager as a way of determining the mass of an exoplanet using nothing more than the signature of its transit.

Now this is interesting stuff because learning something of a planet’s mass can help us make the call on whether it is a rocky world capable of supporting life. Radial velocity methods work well when larger planets are at play, or smaller worlds that orbit extremely close to their parent star. But radial velocity is trickier when we’re dealing with small planets orbiting further out. A planet like the Earth, for example, would be hard to analyze using radial velocity alone. We do, however, have the ability to study planetary atmospheres as planets transit their stars, and ingenious analysis may make it possible to extract from this a reading of a planet’s mass.

Thus de Wit’s description of what the duo have been up to:

“With this method, we realized the planetary mass — a key parameter that, if missing, could have prevented us from assessing the habitability of the first potentially habitable Earth-sized planet in the next decade — will actually be accessible, together with its atmospheric properties.”

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Image: Artistic rendering of a planet’s transmission spectrum. Credit: MIT.

How? Our old friend HD 189733b comes into play, a transiting ‘hot Jupiter’ some 63 light years away that has been a testbed for the technique called transmission spectroscopy, where scientists analyze the light that passes through the atmosphere to determine properties like temperature and the density of atmospheric molecules. Because we can study an atmosphere, we can study the effect of mass on that atmosphere. The method described in the de Wit/Seager paper works with a standard equation that describes the effect of three factors — temperature, gravitational force and atmospheric density — on the planet’s atmospheric pressure profile, which is a measure of how pressure changes throughout the atmosphere.

Believing that each of these factors could be derived independently from a transmission spectrum, de Wit studied the effects of each using the 18th Century mathematical constant called the Euler-Mascheroni constant. This MIT news release describes the constant as an ‘encryption key’ that helped decode the ways the properties of a planet’s atmosphere are embedded in its transmission spectrum. Turned on HD 189733b, the analysis yielded the same mass measurement that other scientists had obtained by radial velocity methods. Future space-based platforms should be able to turn these methods to much smaller worlds.

My particular interest in red dwarfs is piqued by this study, because radial velocity methods are not well suited for small planets orbiting faint stars. Extending the range of mass measurement through this new transit technique would make the mass of planets transiting red dwarfs that much more discoverable, offering another way of characterizing possibly habitable worlds. The technique is not, of course, limited to red dwarfs, and we can assume that Earth-sized planets orbiting stars not so different from the Sun will be studied using the same methods, once we have space-based instruments like the James Webb Space Telescope operational.

The paper is de Wit and Seager, “Constraining Exoplanet Mass from Transmission Spectroscopy,” Science 20 December 2013 (abstract).

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{ 13 comments… read them below or add one }

djlactin December 24, 2013 at 9:47

To all Centaurists: A happy solstice and a felicitous perihelion!

Wojciech J December 24, 2013 at 12:22

Happy Christmas to you as well and all fellow readers and commentators as well.
Reading the blog I couldn’t miss this part
“The technique is not, of course, limited to red dwarfs, and we can assume that Earth-sized planets orbiting stars not so different from the Sun will be studied using the same methods, once we have space-based instruments like the James Webb Space Telescope operational”
Which reminds me, are we still going to have an article next year about membrane telescopes(unless I missed it).I think it was mentioned once.
Recent articles on DARPA’s MOIRA reminded me of this(note telling comparison to JWST size)
http://www.wired.com/dangerroom/2013/12/giant-folding-satellite/

Paul Gilster December 24, 2013 at 12:37

Glad you reminded me about the membrane-telescopes — lots of great work going into all this, and I did mean to have an article out before now. So yes, expect something early in 2014. Ideally, I can get Joe Ritter (UH-Maui) to write this up, but we’ll get it done one way or another.

andy December 24, 2013 at 16:08

On a similar subject, this one came up on the arXiv today:

Betremieux and Kaltenegger, “Impact of atmospheric refraction: How deeply can we probe exo-Earth’s atmospheres during primary eclipse observations?

Food for thought when we consider how to charaterise these distant planets.

In any case, a belated Merry Saturnalia to everyone, and enjoy the holiday season!

Paul Gilster December 24, 2013 at 16:51

I’ve been talking to Lisa Kaltenegger for a couple of months now about writing this work up for us on Centauri Dreams, so we may get a closer look at it soon.

Daniel December 24, 2013 at 17:00

Merry Christmas to all and may you all have a joyous Holiday with friends and family!

Mundus Gubernavi December 24, 2013 at 19:29

Hello all,

Thanks Paul and I hope you enjoy your holiday.

Maybe the Centauri Dreams readers can tap into their gift giving spirit and help out this little 6 year old boy’s dreams:

http://whnt.com/2013/12/09/6-year-old-boy-starts-online-petition-to-save-nasa/

https://petitions.whitehouse.gov/petition/increase-nasa-funding-so-we-can-discover-new-worlds-protect-us-danger-and-make-dreams-come-true-cj/1Qq31jDb

Christopher Phoenix December 25, 2013 at 1:29

Thanks for giving us a heads-up on this rather interesting and exciting new work- without this blog I would have a much harder time keeping up on such news. Any new techniques towards determining the mass of potentially habitable exoplanets is an important step toward determining the actual conditions that exist there. I’l be watching this space for sure!

Oh, and Happy Christmas Eve, Paul!

Tom Baty December 26, 2013 at 14:52

I was just curious, has there been any news, plus or minus about the existence of the possible planet around Alpha Centauri B. Have not seen anything about it for quiet some time.??????

Geoffrey Hillend December 26, 2013 at 16:42

I understand the idea that if we have the pressure profile of an exoplanet’s atmosphere we can derive the mass since mass determines the amount of atmosphere a planet can have, but I would like to know in detail how they determine the pressure profile from spectroscopy.

Intuitively, I think is has to do with the motion of the planet across the disk of the it’s star and the frequency of light changes just before the light is blocked by the exoplanet; We see this when the planet’s atmosphere is the only thing transiting the star just before the surface of the planet begins to transit or cross in front of it’s star?

Geoffrey Hillend December 26, 2013 at 19:36

The idea I was thinking was the that frequency of light changes as the light of it’s star passes through the top part of the planets atmosphere to or thinnest part to the bottom or thickest part which is what they mean by “which is a measure of how pressure changes throughout the planets atmosphere”?

Geoffrey Hillend December 26, 2013 at 19:38

How the rate of time of the star dims as the light passes through the atmosphere?

Michael December 28, 2013 at 9:52

We could possibly also look at the light spectrum for signs of polarisation which could indicate an Ocean as water has the effect of polarising the light reflected off it.

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