NASA's Spitzer Space Telescope has detected infrared light emanating from 55 Cancri e, a dark, blazing-hot planet only twice the size of Earth and eight times as heavy. This marks the first time that light has been detected from a planet of such a small size, and the find is telling astrophysicists where to look in their search for signs of life on planets beyond our own.
55 Cancri e was first discovered in 2004 as one of five celestial bodies orbiting the eponymous 55 Cancri star. It is one of only a handful of known "super-Earths" – planets with a mass up to ten times that of Earth – to cross the face of their star from our vantage point, which allows us to learn more about its nature.
Using infrared imagery, Spitzer has gathered that 55 Cancri e is a dark, desolate, and excruciatingly hot place – something to be expected from a planet 25 times closer to its star than Mercury is to the Sun.
The new data also reinforces the theory that 55 Cancri e might be a waterworld, though not the kind that Kevin Costner could thrive in. The planet appears to have a rocky core surrounded by a layer of water so hot and at such high pressure that it acts as a potent solvent, melting away rock and generating a thick layer of steam.
The planet is tidally locked to its star, with a permanently lit hemisphere in which temperatures reach 1,700 degrees Celsius (3,000ºF), which is hot enough to melt most metals. It transits in front of its star once every 18 hours.
"With such a short period, the transit probability was interestingly high, so we used Spitzer to search for the transit and found it," Michaël Gillon of the Université de Liège in Belgium, the principal investigator of the research, told Gizmag. "Basic assumptions on the albedo and heat distribution efficiency of the planet led us to the conclusion that Spitzer could also detect the occultation of the planet, and thus measure its thermal emission."
As might be expected, the dimming that was registered as the planet transited behind the star was quite small, at only 120 parts per million – in other words, the amount of infrared light coming from the planet was only 0.012 percent that of its star. This was by far the smallest dimming detected for an exoplanet using Spitzer, and marks the first time that any telescope has been able to detect light emanating from a planet as small as a super-Earth.
Other space telescopes, such as Kepler, have detected dimmings ten times fainter than the one found here by Spitzer. However, Kepler observes in the visible spectrum, measuring not the planet's thermal emission, but rather the amount of light that is being reflected in the Earth's direction. This works well for giant planets, but not for the much smaller super-Earths, whose reflection is at best of only 1 ppm, well out of reach even for Kepler.
Taking those measurements in the infrared spectrum is proving a useful technique to collect data on the faint super-Earths, and is also telling astrophysicists where to look for more. "We could measure such a tiny drop of brightness because 55 Cnc is a very bright star visible with the naked eye, which translates into a better photometric precision compared to typical exoplanet-host stars," Gillon continued. "This means that if we want to study tiny exoplanets in the near-future, we really have to focus on detecting them around the most nearby and brightest stars."
Dr. William Danchi, Spitzer program scientist at NASA, added: "The radiation that is measured is in the infrared, which is sensitive to the composition as well as temperature of the atmosphere of the planet. Spitzer was able to measure such a small diameter planet because it was hot, and hot objects emit exponentially more photons that cool objects. It would be much harder to detect a small, cool planet."
Funded for another two years to carry out research on exoplanets and the evolution of galaxies and our Universe, Spitzer is laying in the groundwork for NASA's upcoming James Webb Space Telescope, which will adopt a similar technique to collect data on potentially habitable planets. Scheduled for launch in 2018, it will likely learn even more about the planet's composition and search other planets for signs of molecules that are the telltale indicators of life.
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