Spitzer measured the extraordinarily small amount of light 55 Cancri e blocked when the planet crossed in front of its star. These mini-eclipses, called transits, allow astronomers to accurately determine a planet's size and calculate its density. Promisingly, the results show how astronomers can use Spitzer, operating in "warm" mode since depleting its liquid coolant in May 2009, to probe the properties of strange alien worlds.
"This work demonstrates that 'warm' Spitzer can measure an extremely faint eclipse caused by exoplanets' transits with very high precision," said Brice-Olivier Demory, a post-doctoral associate in Professor Sara Seager's group in the Earth, Atmospheric and Planetary Sciences department at the Massachusetts Institute of Technology (MIT). Demory, who is lead author of a paper accepted for publication in Astronomy & Astrophysics, said that the study "emphasizes the important role Spitzer still has to play for the detection and characterization of transiting planets."
Blazing Hot and on the Move
Astronomers first discovered 55 Cancri e in 2004, and continued investigation of the exoplanet has shown it to be a truly bizarre place. The world revolves around its sunlike star in the shortest time period of all known exoplanets - just 17 hours and 40 minutes. (In other words, a year on 55 Cancri e lasts less than 18 hours.) The exoplanet orbits about 26 times closer to its star than Mercury, the most Sun-kissed planet in our solar system. Such proximity means that 55 Cancri e's surface roasts at a minimum of 3,200 degrees Fahrenheit (1,760 degrees Celsius).
The new observations with Spitzer reveal 55 Cancri e to have a mass 7.8 times and a radius just over twice that of Earth. Those properties place 55 Cancri e in the "super-Earth" class of exoplanets, a few dozen of which have been found. Only a handful of known super-Earths, however, cross the face of their stars as viewed from our vantage point in the cosmos. At just 40 light years away, 55 Cancri e stands as the smallest transiting super-Earth in our stellar neighborhood. In fact, 55 Cancri is so bright and close that it can be seen with the naked eye on a clear, dark night.
Based on the precise Spitzer data, Demory and his colleagues came up with a revised, lower density for 55 Cancri e. Coupled with its tight orbit, 55 Cancri e possesses a unique combination of super-Earth traits. Its low density is similar to that of a cooler super-Earth called GJ1214b, discovered in 2009 orbiting a tiny, dim star. Yet 55 Cancri e's orbit is more like that of the denser, inferno worlds CoRoT-7b and Kepler-10b. "What makes 55 Cancri e so remarkable is that despite its high temperature, the planet has a low density," said Demory.
Previously, a separate international team of astronomers had made observations of 55 Cancri e in visible light with Canada's MOST telescope. Initially, their evidence implied that 55 Cancri e's diameter was smaller by 25 percent, leading to reports of 55 Cancri e as actually the densest planet known. Refinements to those observations, however, now agree with the new Spitzer findings, which rely on a transit seen in longer-wavelength infrared light.
Exoplanetary Origins and Future Demise
No longer looking like a dense planet of solid rock, 55 Cancri e instead appears to be an unprecedented world with an intriguing history. The Spitzer results suggest that about a fifth of the planet's mass must be made of light elements and compounds, including water. In the intense heat of 55 Cancri e's terribly close sun, those light materials would exist in a "supercritical" state, between that of a liquid and a gas, and might sizzle out of the planet's surface.
New developments in planetary formation and evolution theory will probably be necessary to explain 55 Cancri e's back story. According to our models of the birth of solar systems, for example, 55 Cancri e could not have formed so near its star. Maybe it started out as a more distant planet with a large gaseous atmosphere. As worlds took shape in the 55 Cancri solar system, gravitational interactions amongst the system's five known planets could have prodded a young 55 Cancri e to migrate in toward its sun. In the process, the Neptune-like exoplanet might have lost most of its atmosphere, exposing a core that sputters with the venting of heated chemicals.
It seems certain that 55 Cancri e is on a "death spiral," soon to be devoured or ripped apart by its host star. But for now, the world's serendipitous placement in our sky will allow Spitzer and other instruments to study 55 Cancri e in further detail, expanding our knowledge of how exoplanets work.
"55 Cancri e orbits a very bright star thus enabling the possibility of obtaining a wealth of observations with space-based facilities at various wavelengths," said study co-author Michael Gillon of the University of Liege in Belgium and principal investigator for the warm Spitzer program aimed at detecting transiting low-mass exoplanets. "This fact will make 55 Cancri e a landmark for our understanding of the planetary interior and atmospheric composition of super-Earths."
Other authors of the paper are Diana Valencia, Sara Seager and Bjorn Benneke of MIT; Drake Deming of the University of Maryland; Christophe Lovis, Michel Mayor, Francesco Pepe, Didier Queloz, Damien Ségransan, and Stéphane Udry of the University of Geneva; and Patricio Cubillos, Joseph Harrington, and Kevin B. Stevenson of the University of Central Florida.