Using data from NASA’s Spitzer Space Telescope, astronomers have mapped how conditions on 55 Cancri e – a ‘nearby’ super-Earth exoplanet – change throughout a complete orbit.
Artist’s impression of the super-Earth 55 Cancri e orbiting its host star. Image credit: NASA / JPL-Caltech.
55 Cancri e is one of five planets orbiting the Sun-like star 55 Cancri A, which is approximately 40 light-years away in the constellation Cancer.
This planet was first discovered in August 2004 by McDonald Observatory astronomer Dr. Barbara McArthur and her colleagues.
It has a radius twice Earth’s, and a mass 8 times greater, making it a so-called super-Earth.
55 Cancri e orbits its parent star at a distance of 0.015 AU – about 25 times closer than Mercury is to our Sun – every 18 hours.
The planet is also tidally locked, meaning that it doesn’t rotate like Earth does – instead there is a permanent ‘day’ side and a ‘night’ side.
According to an international team of astronomers led by Cavendish Laboratory scientist Dr. Brice-Olivier Demory, conditions on the hot side of 55 Cancri e are so extreme that it may have caused the atmosphere to evaporate, with the result that conditions on the two sides of the planet vary widely: temperatures on the hot side can reach 4,532 degrees Fahrenheit (2,500 degrees Celsius), while temperatures on the cool side are around 2,012 degrees Fahrenheit (1,100 degrees Celsius).
“We report a longitudinal thermal brightness map of the nearby transiting super-Earth 55 Cancri e revealing highly asymmetric dayside thermal emission and a strong day–night temperature contrast,” the astronomers said.
“Dedicated space-based monitoring of the planet in the infrared revealed a modulation of the thermal flux as 55 Cancri e revolves around its star in a tidally locked configuration. These observations reveal a hot spot that is located around 41 east of the substellar point.”
55 Cancri e has been extensively studied since its discovery. Based on readings taken at different points in time, it was thought to be a water world, or even made of diamond, but Dr. Demory and co-authors believe that the planet is almost completely covered by lava.
The ‘day’ side of 55 Cancri e appears to be almost completely molten, while the ‘night’ side is almost completely solid. The heat from the day side is not efficiently circulated to the night side, however.
On Earth, the atmosphere aids in the recirculation of heat, keeping the temperature across the whole planet within a relatively narrow range. But on 55 Cancri e, the hot side stays hot, and the cold side stays cold.
“One possibility for this variation could be either a complete lack of atmosphere, or one which has been partially destroyed due to the strong irradiation from the nearby host star,” Dr. Demory said.
“We think that there could still be an atmosphere on the night side, but temperatures on the day side are so extreme that the atmosphere may have evaporated completely, meaning that heat is not being efficiently transferred, or transferred at all from the day side to the night side.”
“Another possibility for the huge discrepancy between the day side and the night side may be that the molten lava on the day side moves heat along the surface, but since lava is mostly solid on the night side, heat is not moved around as efficiently.”
What is unclear however, is where exactly the ‘extra’ heat on 55 Cancri e comes from in the first place, since the observations reveal an unknown source of heat that makes the planet hotter than expected solely from the irradiation from the star – but the researchers may have to wait until the next generation of space telescopes are launched to find out.
Dr. Demory and co-authors plan to keep studying 55 Cancri e, in order to see what other secrets it might hold, including the possibility that it might be surrounded by a torus of gas and dust, which could account for some of the variations in the data.
The findings were published online this week in the journal Nature.
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Brice-Olivier Demory et al. A map of the large day–night temperature gradient of a super-Earth exoplanet. Nature, published online March 30, 2016; doi: 10.1038/nature17169
