The star K2-18 has two super-Earths of which one is potentially habitable

Artist's concept of part of the planet K2-18b with a thick atmosphere, its star and the planet K2-18c in the background (Image courtesy Alex Boersma)
Artist’s concept of part of the planet K2-18b with a thick atmosphere, its star and the planet K2-18c in the background (Image courtesy Alex Boersma)

An article published in the journal “Astronomy & Astrophysics” describes a research on the star K2-18’s system. A team of researchers used the HARPS instrument at ESO’s La Silla observatory in Chile to study the exoplanet K2-18b, discovered in 2015, which could be a larger version of the Earth. The analysis of the data led to the discovery of a second exoplanet, which was called K2-18c, a little less massive and closer to its star therefore probably too hot to be in ​​its system’s habitable zone.

About 111 light years away from Earth, the star K2-18 is a red dwarf with a mass that is about 40% of the Sun’s. This means that it’s much colder than the Sun, with the consequence that a planet must be much closer to it than the Earth is to the Sun to receive a similar amount of energy. In fact, K2-18b is much closer to its star than Mercury is to the Sun and its year lasts only 33 Earth days but when it was discovered in 2015 it raised interest precisely because it’s in ​​its system’s habitable zone.

Being an interesting candidate as a potentially habitable exoplanet, K2-18b was studied using the HARPS (High Accuracy Radial Velocity Planet Searcher) instrument, useful for both the research of new exoplanets and to obtain more information on the ones already known. The examination of the data allowed to estimate this planet’s mass, even if in a fairly approximate way, between a little over 6 and almost 10 times the Earth’s. Its size was already known quite precisely with a radius measured at about 2.24 times the Earth’s.

These results leave a certain degree of ambiguity in the interpretation making it difficult to establish with certainty the planet K2-18b’s type. It’s a super-Earth that could have a thick gas atmosphere but it could also be a so-called ocean world, completely covered by an ocean of considerable depth.

Before this study there was the possibility that K2-18b was a mini-Neptune but the set of data collected rules it out. In fact, its estimated mass is 3 times if not 4 times that of Kepler-11f, one of the smallest known gas planets, which has a size slightly greater than that of K2-18b. Basically, the K2-18b’s density is that of a rocky planet.

This research also allowed to discover another super-Earth in that star system, which was called K2-18c. It was a nice surprise and Ryan Cloutier, the article’s lead author, explained that it came after he and his team conducted a series of data checks to make sure it was really another planet.

The size and mass of the exoplanet K2-18c seem slightly lower than K2-18b’s but its distance from their star is about 60% of K2-18b’s. That star is a red dwarf but K2-18c is probably too close to it because on its surface there can be conditions similar to those of the Earth.

This new discovery is still interesting because it’s another case of a small star with various planets, a crucial factor for research since theose star are the most common and consume their hydrogen slowly so their systems can last very long. The doubts are connected to their flare that can flood their planets with large amounts of radiation but the potential deserves the increasing amount of studies conducted in recent years.

In this specific case, the exoplanet K2-18b will certainly be the subject of further examinations. Ryan Cloutier stated that the doubts about it could be solved by observing it with the James Webb space telescope, now scheduled for launch in early 2019, which will allow to study its atmosphere as well.

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