Astronomer’s Map a ‘Super-Earth’ That Is Half Lava, Half Solid Rock

April 1, 2016 | Joanne Kennell

Illustration of 55 Cancri e
Photo credit: NASA/JPL-Caltech

A mysterious heat source is making the planet hotter than it should be.

An international team of astronomers, led by the University of Cambridge, have obtained an extremely detailed ‘fingerprint,’ the most comprehensive to date, of a rocky planet outside our solar system. To their surprise, they discovered it is a planet of two worlds: one half is almost completely molten — the hot side — and the cool side is nearly completely solid.

According to the researchers, conditions on the hot side of the planet are so extreme that it may have resulted in the evaporation of the atmosphere. The temperature on the hot side can reach 2,500 degrees Celsius (4,530 degrees Fahrenheit), while temperatures on the cool side are around 1,100 degrees Celsius (2,010 degrees Fahrenheit).

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Using data from NASA’s Spitzer Space Telescope, the researchers examined 55 Cancri e, which orbits a sun-like star located 40 light-years away in the Cancer constellation, and they mapped how conditions on the planet change throughout a complete orbit.

55 Cancri e is a ‘super Earth’ — a rocky exoplanet twice the size and eight times the mass of Earth — which orbits its star so closely that a year lasts just 18 hours. The planet is also tidally locked, meaning it always shows the same face to its parent star, similar to Earth’s moon where there is a permanent ‘day’ and ‘night’ side.

However, observing the characteristics of super Earths is difficult since they are usually very small compared to their parent star.

“We haven't yet found any other planet that is this small and orbits so close to its parent star, and is relatively close to us, so 55 Cancri e offers lots of possibilities,” Dr. Brice-Olivier Demory of the University's Cavendish Laboratory and the paper's lead author said in a press release. “We still don't know exactly what this planet is made of — it's still a riddle. These results are like adding another brick to the wall, but the exact nature of this planet is still not completely understood.”

55 Cancri e was discovered in 2011. It was previously thought to be a water world, or even made of diamond, however researchers now believe that it is almost completely covered by lava.

“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, in the release. “It is incredible that we are now able to measure the large scale temperature distribution on the surface of a rocky exoplanet.”

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What is so interesting about this planet is how vastly different each side is, and scientists are not exactly sure why there is such a huge difference in temperature. For some reason, the heat from the ‘day’ side is not efficiently circulated to the ‘night’ side. On Earth, the atmosphere aids in the recirculation of heat, keeping the temperature across the planet relatively equal.

According to Demory, one possibility could be either a complete lack of atmosphere on the day side, or one which has been partially destroyed due to the strong irradiation from its star. Another possibility is that the molten lava on the day side moves heat along the surface, but since the lava is mostly solid on the night side, heat is not moved around as efficiently.

However, another mystery lies in where exactly the ‘extra’ heat on 55 Cancri e is coming from. The researchers’ observations revealed that an unknown source of heat is making the planet hotter than expected, based solely on the irradiation from its star. Unfortunately, the astronomers have to wait until the next generation of space telescopes are launched to figure that out.

According to Demory, these readings show just how hard it is to detect a planet that is similar to Earth. However, “understanding the surface and climate properties of these other worlds will eventually allow us to put the Earth's climate and habitability into context.”

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