Unusual atmospheric data from this exoplanet: not much heat transfer from the side permanently facing its star to the dark side, giving it a ‘large day–night temperature gradient’.
The orbit period is only 18 hours, as it’s much nearer to its star than Mercury is to the Sun. It may also have ‘an unknown source of heat’, as Phys.org reports.
An international team of astronomers, led by the University of Cambridge, has obtained the most detailed ‘fingerprint’ of a rocky planet outside our solar system to date, and found a planet of two halves: one that is almost completely molten, and the other which is almost completely solid.
According to the researchers, conditions on the hot side of the planet 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 2500 degrees Celsius, while temperatures on the cool side are around 1100 degrees.
The results are reported in the journal Nature. Using data from NASA’s Spitzer Space Telescope, the researchers examined a planet known as 55 Cancri e, which orbits a sun-like star located 40 light years away in the Cancer constellation, and have mapped how conditions on the planet change throughout a complete orbit, the first time this has been accomplished for such a small planet.
55 Cancri e is a ‘super Earth’: a rocky exoplanet about twice the size and eight times the mass of Earth, and orbits its parent star so closely that a year lasts just 18 hours. The planet is also tidally locked, meaning that it always shows the same face to its parent star, similar to the Moon, so there is a permanent ‘day’ side and a ‘night’ side. Since it is among the nearest super Earths whose composition can be studied, 55 Cancri e is among the best candidates for detailed observations of surface and atmospheric conditions on rocky exoplanets.
“We have entered a new era of atmospheric remote sensing of rocky exoplanets,” said study co-author Dr Nikku Madhusudhan, from the Institute of Astronomy at Cambridge. “It is incredible that we are now able to measure the large scale temperature distribution on the surface of a rocky exoplanet.”
Based on these new infrared measurements, the ‘day’ side of the planet 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.
According to Demory, 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. “On the day side, the temperature is around 2500 degrees Celsius, while on the night side it’s about 1100 degrees – that’s a huge difference,” he 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.
Phys.org report: Map of rocky exoplanet reveals a lava world
‘Nature’ preview : A map of the large day–night temperature gradient of a super-Earth exoplanet