An international team of astronomers has found that a little-known exoplanet called K2-18b could be a scaled-up version of Earth, and also discovered the planet has a massive neighbor, K2-18c.
This artist’s concept shows two super-Earth exoplanets, K2-18b and c, orbiting the red dwarf star K2-18. Image credit: Alex Boersma.
“Being able to measure the mass and density of K2-18b was tremendous, but to discover a new exoplanet was lucky and equally exciting,” said Ryan Cloutier, a PhD student at the University of Toronto and the Université de Montréal Institute for Research on Exoplanets.
Both exoplanets orbit K2-18 (also known as EPIC 201912552), a red dwarf star located approximately 111 light-years away in the constellation Leo.
When K2-18b was first discovered in 2015, it was found to be orbiting within the star’s habitable zone, making it an ideal candidate to have liquid surface water.
In order to figure out whether the planet was a scaled-up version of Earth (mostly rock), or a scaled-down version of Neptune (mostly gas), Cloutier and co-authors had to first figure out the planet’s mass.
“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,” Cloutier said.
The astronomers found that K2-18b has a mass about 8 times that of Earth.
It has a bulk density of 3.7g/cm3 and is either a mostly rocky planet with a small gaseous atmosphere — like Earth, but bigger — or a mostly water planet with a thick layer of ice on top of it.
“It was while looking through the data of K2-18b that we noticed something unusual,” he said.
In addition to a signal occurring every 39 days from the rotation of K2-18, and one taking place every 33 days from the orbit of K2-18b, they noticed a different signal occurring every 9 days.
“When we first threw the data on the table we were trying to figure out what it was,” Cloutier said.
“You have to ensure the signal isn’t just noise, and you need to do careful analysis to verify it, but seeing that initial signal was a good indication there was another planet.”
The newfound alien world, K2-18c, is also a super-Earth, with a mass about 7.5 times that of Earth.
The planet is closer to its star than K2-18b, and probably too hot to be in the habitable zone.
“It wasn’t a Eureka! moment because we still had to go through a checklist of things to do in order to verify the data,” Cloutier noted.
“Once all the boxes were checked it sunk in that, wow, this actually is a planet.”
A paper describing the discovery will appear in the journal Astronomy & Astrophysics and is now available online at arXiv.org.
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R. Cloutier et al. 2017. Characterization of the K2-18 multi-planetary system with HARPS: A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit. Astronomy & Astrophysics, in press; arXiv: 1707.04292
