The exoplanet EPIC 228732031b is very close to its star

Artist's concept of ultra-short-period exoplanet and its star (Image NASA’s Goddard Space Flight Center / TESS / MIT / Lincoln Laboratory)
Artist’s concept of ultra-short-period exoplanet and its star (Image NASA’s Goddard Space Flight Center / TESS / MIT / Lincoln Laboratory)

An article accepted for publication in “The Astronomica Journal” describes the discovery of a planet that orbits EPIC 228732031, a star just a little smaller than the Sun. A team of researchers used NASA’s Kepler space telescope to detect traces of the transits of the exoplanet that was called EPIC 228732031b. This type of discovery has become common but in this case it’s a super-Earth whose orbit is very close to its star, so much that its year only lasts 8.9 hours.

Approximately 568 light years away from Earth, the star EPIC 228732031, also known as K2-131, 2MASS J12110036-0945547 or WISE J121100.34-094554.7, is similar to the Sun since it has about 84% of its mass and a surface temperature estimated at about 5,200 Kelvin, more or less 500 Kelvin lower than that on the Sun’s surface.

Thanks to these similarities it’s easy to make comparisons between the two solar systems and understand that a planet that orbits the star EPIC 228732031 in only 8.9 hours is so close to it that the conditions on its surface are probably hellish with temperatures that could reach even 3,000 Kelvin on the side exposed to the star.

A planet so close to its star is certainly tidally locked, meanning that the same hemisphere is always facing its star like the Moon to the Earth. This means that on the side of EPIC 228732031b facing its star the surface is made of lava while on the opposite could be solid at least in regions far enough from the lava.

In the course of the search for exoplanet, others have already been found with orbital periods less than an Earth’s day. They have been grouped into a specific class called ultra-short-period. They include various types of planets: EPIC 228732031 is a super-Earth and therefore a rocky planet but there are also cases of hot Jupiters and therefore gas giants.

A super-Earth belonging to that category is Kepler-78b, dubbed the Earth’s infernal twin because it’s slightly larger than the Earth and orbits its star in just 8.5 hours. Its star is a little smaller than EPIC 228732031 but its surface temperature is just slightly lower so on the hemisphere of the planet Kepler-78b facing it the surface is melted as well.

The rocky ultra-short-period exoplanets typically also fall into the category of lava planets. It also includes very young planets that are still solidifying and those that suffered a violent heating due to the impact with an object large enough to cause the melting of a significant part of its surface.

An interesting characteristic found in ultra-short-period exoplanets is that in many cases other planets have been found in a nearby orbit. Right now no other planets orbiting the star EPIC 228732031 were found but the search continues. It’s unclear whether this characteristic is a coincidence and there’s still no clear explanation for the existence of exoplanets so close to their stars. They’re likely to have formed farther and then got closer but in the long term this could result in their destruction.

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