Illustration courtesy Caltech/NASA
Published May 10, 2012
The light from Jupiter-like gas giants has been seen before. But the new finding proves that researchers can now detect the infrared glow from smaller, more Earthlike worlds.
Dubbed 55 Cancri e, the planet lies roughly 40 light-years away in the constellation Cancer, the crab. Astronomers call it a super-Earth because even though the planet is about twice the size of Earth and about eight times as massive, it could have a rocky surface, like our home world.
However, 55 Cancri e isn't likely to be habitable.
That's because this super-Earth orbits its star closer than Mercury circles the sun. What's more, one side of the planet always faces its host star, so that its "day side" is a scorching 3,700 degrees Fahrenheit (2,037 degrees Celsius)—hot enough to melt steel.
Still, other super-Earths with more friendly conditions could now be targets for future infrared studies.
"One of the best ways to get information on a planet is to study the light that's emitted ... and to do that you have to be able to detect the light in the first place," said planetary scientist Heather Knutson of the California Institute of Technology, who wasn't involved in the new study.
"This observation tells us a little about 55 Cancri e, but the really exciting thing is that it shows this kind of measurement is possible for these kinds of planets. That's the kind of crack of the door that we were looking for."
Seeing the Light
The extrasolar planet—or exoplanet—55 Cancri e was discovered in 2004 using a technique called radial velocity, which measures how much a star "wobbles" due to the gravitational tug of orbiting worlds.
Seeing the planet's light directly, though, was more of a challenge, because the relatively small world is largely drowned out by the glare from its bright host star.
To unmask the planet, Brice-Olivier Demory and Sara Seager of the Massachusetts Institute of Technology used Spitzer to watch the star during an occultation—when the planet passes behind its host.
(Related: "Pluto to Make a Star 'Wink Out' Twice.")
By measuring starlight before and after the occultation, the team could detect a tiny drop in brightness when the planet was totally eclipsed by its star.
"When you stack all the data together, you see a beautiful dimming light that clearly shows the light from the planet that disappears," said study co-author Michael Gillon, principal investigator of the Spitzer program.
MIT's Seager added, "The same technique has been used for the hot Jupiters"—gas giants that closely orbit their stars—"but this is different because the planet is so much smaller."
The new Spitzer data seem to confirm earlier theories of the exotic planet, which painted 55 Cancri e as a dark-colored world with a rocky core, a layer of "supercritical" water—when water is under such extreme temperatures and pressures that it acts like both a liquid and a gas—and an overlying blanket of steam.
Tip of the Iceberg
Now that scientists have demonstrated how to use Spitzer to spot and characterize small planets, they'll be able to use similar techniques to explore other super-Earths.
"Each new break point in exoplanets is always the tip of the iceberg, with many more to follow," Seager told National Geographic News.
"In this case, we have shown that the Spitzer Space Telescope is very stable, and many observations can be stacked together to reach smaller and smaller planets.
"This bodes well for future studies of smaller, cooler planets that might be considered more interesting than 55 Cancri e."
The new super-Earth study appears in the current issue of Astrophysical Journal Letters.
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