The exoplanet, called K2-18b, which orbits the star K2-18, could well be a scaled-up version of Earth, according to the study to be published in the journal Astronomy and Astrophysics.
While looking at the planet's data, the same researchers also discovered for the first time that the planet has a neighbour.
"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 lead author Ryan Cloutier from University of Toronto.
When the planet 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, a key element in harbouring conditions for life as we know it.
The data set used by the researchers came from the High Accuracy Radial Velocity Planet Searcher (HARPS) using the European Southern Observatory's 3.6m telescope at La Silla Observatory, in Chile.
In order to figure out whether K2-18b was a scaled-up version of Earth (mostly rock), or a scaled-down version of Neptune (mostly gas), researchers had to first figure out the planet's mass, using radial velocity measurements taken with HARPS.
"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.
After using a machine-learning approach to figure out the mass measurement, the researchers were able to determine the planet 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.
"But with the James Webb Space Telescope (JWST) we can probe the atmosphere and see whether it has an extensive atmosphere or it's a planet covered in water," he added.
The JWST, which will be launched in 2019, could be instrumental in collecting a range of data for studying the solar system, early universe and exoplanets.
It was while looking through the data of K2-18b that Cloutier noticed something unusual.
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, he noticed a different signal occurring every nine days.
"When we first threw the data on the table we were trying to figure out what it was. 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," Cloutier said.
While the newly described planet K2-18c is closer to its star, and probably too hot to be in the habitable zone, like K2-18b it also appears to be a Super-Earth meaning it has a mass similar to Earth.
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