“Spitzer has amazed us yet again,” said Bill Danchi from NASA Headquarters in Washington, D.C. “The spacecraft is pioneering the study of atmospheres of distant planets and paving the way for NASA’s upcoming James Webb Space Telescope to apply a similar technique on potentially habitable planets.”
The planet, called 55 Cancri e, falls into a class of planets termed super-Earths, which are more massive than our home world but lighter than giant planets like Neptune. The planet is about twice as big and eight times as massive as Earth. It orbits a bright star, called 55 Cancri, in a mere 18 hours.
Previously, Spitzer and other telescopes were able to study the planet by analyzing how the light from 55 Cancri changed as the planet passed in front of the star. In the new study, Spitzer measured how much infrared light comes from the planet itself. The results reveal that the planet is likely dark, and its sun-facing side is more than 3140° Fahrenheit (1725° Celsius), hot enough to melt metal.
The new information is consistent with a prior theory that 55 Cancri e is a water world — 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.
“It could be very similar to Neptune, if you pulled Neptune in toward our Sun and watched its atmosphere boil away,” said Michaël Gillon from the University of Liège in Belgium.
The 55 Cancri system is relatively close to Earth at 41 light-years away. It has five planets, with 55 Cancri e the closest to the star and tidally locked so one side always faces its sun. Spitzer discovered that the sun-facing side is extremely hot, indicating the planet probably does not have a substantial atmosphere to carry the star’s heat to the unlit side.
NASA’s James Webb Space Telescope, scheduled to launch in 2018, likely will be able to learn even more about the planet’s composition. The telescope might be able to use a similar infrared method to Spitzer to search other potentially habitable planets for signs of molecules possibly related to life.
“When we conceived of Spitzer more than 40 years ago, exoplanets hadn’t even been discovered,” said Michael Werner from NASA’s Jet Propulsion Laboratory in Pasadena, California. “Because Spitzer was built very well, it’s been able to adapt to this new field and make historic advances such as this.”
In 2005, Spitzer became the first telescope to detect light from a planet beyond our solar system. To the surprise of many, the observatory saw the infrared light of a “hot Jupiter,” a gaseous planet much larger than the solid 55 Cancri e. Since then, other telescopes, including NASA’s Hubble and Kepler space telescopes, have performed similar feats with gas giants using the same method.
In this method, a telescope gazes at a star as a planet circles behind it. When the planet disappears from view, the light from the star system dips ever so slightly, but enough that astronomers can determine how much light came from the planet itself. This information reveals the temperature of a planet, and, in some cases, its atmospheric components. Most other current planet-hunting methods obtain indirect measurements of a planet by observing its effects on the star.
During Spitzer’s ongoing extended mission, steps were taken to enhance its unique ability to see exoplanets, including 55 Cancri e. Those steps, which included changing the cycling of a heater and using an instrument in a new way, led to improvements in how precisely the telescope points at targets.
