Scientists have identified a distant “super-Earth” that has a potential for hosting alien life, but that’s not all. This exoplanet, K2-18b, has another planet lurking just behind it.
Artist's impression of the K2-18 star system [Image by Alex Boersma]
As far as exoplanets go, K2-18b certainly isn’t the most well-known, like the TRAPPIST-1 planets or Proxima b. K2-18b was discovered in 2015, orbiting in the habitable zone of a red dwarf around 111 light years away from Earth in the constellation Leo. Because K2-18 was orbiting in the habitable zone of its system, scientists thought that it may have had liquid water on its surface, which may indicate that life may thrive on the planet. However, newer observations have revealed that K2-18b also has a close neighbor that telescopes missed.
While the discovery that K2-18b is possibly habitable is exciting, so is the discovery that it has a neighbor similar to it. Both planets are super-Earths, or planets more massive than Earth but less so than planets like Jupiter or Neptune.
The initial discovery of K2-18b wasn't met with much fanfare [Image by M. Weiss, CfA]
According to a recent study, the High Accuracy Radial Velocity Planet Searcher (HARPS) provided the data set used by researchers to discover the presence of the second Earth-like planet. HARPS, as its name suggests, is used to hunt for planets as far as the device can look.
HARPS technology also helped researchers determine if K2-18b was a rocky planet like Earth, or a gassy one like Jupiter. The same technology also allowed researchers to figure out the size of the planet by analyzing the star K2-18’s radial velocity data. "If you can get the mass and radius, you can measure the bulk density of the planet and that can tell you what the bulk of the planet is made of," said Ryan Cloutier, the lead author of the study. Eventually, the researchers were able to figure out that K2-18b could be mostly rocky, with a gaseous atmosphere. There’s also a possibility that it could be a watery planet with an icy crust.
Cloutier says that given the current data, they can’t tell which of the two possibilities is more possible. "But with the James Webb Space Telescope (JWST),” Cloutier continues, “we can probe the atmosphere and see whether it has an extensive atmosphere or it's a planet covered in water."
Red dwarfs typically host potentially habitable planets [Image by ESO]
Strange readings on K2-18b’s data clued researchers in to the existence of another planet lurking around K2-18. K2-18’s rotation signal occurred every 39 days, while K2-18b’s orbit signal occurred every 33 days. However, Cloutier detected another signal, which came up every nine days. He figured that this third signal is a very good indication that there was another planet in the vicinity.
This other planet has been designated K2-18c. It’s closer than K2-18b to their host star, probably too close to be in the habitable zone. Observations indicate that K2-18c is also likely a super-Earth. Its discovery and identification as a super-Earth, the second one in a single star system, contributes to current knowledge on red dwarf stars with multiple planets.
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