Tuesday, May 8, 2012

Astronomy: Spitzer Sees Light from Super Earth

The Spitzer Space Telescope is an orbiting satellite that takes data at near and mid infrared wavelengths. It had to be cryogenically cooled, otherwise it would detect its own heat in its instruments! Right now, the cryogen has run out so it only operates at the shortest wavelengths (3.5 and 4.6 microns), but is still useful for a lot of science ranging from brown dwarfs to exoplanets to dusty galaxies.

Today, NASA has revealed that Spitzer has detected the light from a distant super-Earth; the first ever such observed. At first I was surprised, since I had heard about Spitzer imaging a planet before (HD 209458b and TrES-1, see here), but it turns out that these were gas giant planets, not a super Earth.

What is a super Earth?
Very simply, it is a term used to refer to planets that aren't massive or large enough to be considered giant planets like Jupiter or Neptune, but are much larger than the Earth. Most planets found in the first decade of planet searches were hot Jupiters: giant planets the size of Jupiter (or larger) orbiting very close to their stars. Now we are finding more low-mass planets and have reached the interesting range of mini-Neptunes and super-Earths. The distinction is a bit arbitrary, but super-Earths are presumed to be rocky (like Earth) whereas mini-Neptunes are presumed to be more like gas/ice giants (ie, like Neptune).

What did Spitzer do?
Spitzer, or more accurately the scientists involved in the paper: Michaël Gillon (of Université de Liège in Belgium), Brice-Olivier Demory (of the Massachusetts Institute of Technology in Cambridge), and others in the team, targeted 55 Cancri, a nearby star 41 light-years away known to host several planets. They timed their measurements so they would observe the system when the planet passed behind the star rather than in front. When a planet passes in front of the star we have a transit (like the Venus transit this June) and the star grows dimmer. When a planet passes behind the star we have an occultation. In general, the system appears the same brightness; however, if the planet was warm enough, it would contribute to the total light and thus the system's brightness would again decrease (by a small amount) in an occultation. The advantage to using Spitzer is that it measures infrared light and planets can contribute more at infrared wavelengths than at optical ones (visible light) to the total system.

This is what was observed for the occultation of 55 Cancri e:
The light of the star+planet system decreased when the planet passed behind the star. The transit of the planet, which is far deeper, is not shown as it happens near orbital phase 0. Credit: NASA/JPL-Caltech/MIT

What the figure above is showing is the 4.5 micron infrared emission of the total system around the time of the occultation. When the occultation starts (around orbital phase 0.45) the light of the system drops by a very small, but detectable, amount. The missing light is coming from the planet and since it has gone behind the star it's no longer adding its glow to the total light of the system.

55 Cancri e
The planet in question is the 4th planet (of 5 known) discovered around 55 Cancri and the closest in the system. It lies far closer than Mercury does to the Sun in fact. It's orbit is only about 18 hours (Mercury's is 88 days). As such, 55 Cancri e is scorched by the light of its sun-like star and glows in infrared light. The dayside temperatures can reach 2000 Kelvins (3140 degrees Fahrenheit): hot!
The models, surprisingly, show it may be a water world in that it consists of "a rocky core surrounded by a layer of water in a supercritical state where it is both liquid and gas, and topped by a blanket of steam." Not what I usually think of as a "water world."

55 Cancri e, orbiting its star. Credit: NASA/JPL-Caltech

The system itself is a binary star, but the secondary, a faint red dwarf, is over 1000 AU away from the primary sun-like star and presumably has had little to no influence on the formation and evolution of the planets in the system. It's not the first case we see planets in binary systems (see here and here for prior blog posts on this).

Final Thoughts
Spitzer has previously detected light from hot Jupiter being occulted by their host stars. This is the first case in which this has been done for a super Earth system.
Why is this exciting? These type of planets are only a little larger than Earth, and while 55 Cancri e isn't capable of sustaining life as we know it, the study of planets like it will let us understand the properties of super Earths and Earths alike.

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