Exoplanet could be 'super-Earth,' Canadian astronomers say

Canadian astronomers have not only discovered that a world orbiting a star 111 light years away could be a "super-Earth," they've also discovered a new exoplanet in the same solar system. 

The newly discovered world was found when a team of researchers, led by Ryan Cloutier, a PhD student at the University of Toronto's department of astronomy and astrophysics, was analyzing data collected from the European Southern Observatory (ESO) on the K2-18 star system which was found to have a planet — K2-18b — orbiting in 2015.

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The new data suggests K2-18b is either a large, rocky planet, like Earth, or one with mostly ice and water on its surface. It's what astronomers call a "super-Earth" — a new but vaguely defined class of exoplanets whose size and mass are between those of Earth and Neptune, which is about four times larger than our planet.

"These are actually the most common type of planet in the universe," Cloutier told CBC News. "Things that are between the size of Earth and Neptune are more common than Earths and more common than Neptunes."

If these super-Earths orbit in the habitable zone — an area where liquid water can exist on the surface — they could be rocky worlds with oceans, and possibly life.

K2-18b's orbit takes it around its red M-dwarf star (one of the most common types of stars) every 33 days, meaning it's about 15 per cent the distance from its star as we are from the sun. It's about 2.5 times bigger than Earth, but eight times more massive.  

K2-18c — the newly discovered planet — has roughly the same size and mass. However, it orbits closer to its host star, about 60 per cent the distance of K2-18b. For that reason, it's unlikely to be a potentially habitable world.

Uncovering the new world

One of the more common methods of finding exoplanets is by their transit. A space telescope, like Kepler, measures a star's brightness. If it dims by a tiny fraction, that indicates that a planet has most likely crossed — or transited — between the star and the observer.

In the case of the ESO, the data was collected using the High Accuracy Radial Velocity Planet Searcher (HARPS) on a telescope in Chile. While studying a transit can reveal the existence and size of an exoplanet, studying the radial velocity — the tiny wobbles of the star — can shed light on its mass and, in turn, density.

It was while studying those tiny movements of the star that something interesting was discovered: the star wasn't wobbling as it should if there was just one planet. There was a indication in the data that arose every nine days.

"So you go, what the heck's that thing at nine days?" Cloutier said.

After eliminating other possibilities, such as activity on the star itself, it was clear there was another planet orbiting the star. Kepler was unable to spot it as it was on an inclined plane and didn't pass in front of the star. 

It was an exciting discovery for Cloutier who said he had always hoped to discover an exoplanet.

"The most thrilling part was that first hint that there might be something there," said Cloutier. "That was more exciting than when it got confirmed."

Unravelling the mysteries

René Doyon, Cloutier's co-supervisor and a co-author of the paper published Tuesday in the journal Astronomy and Astrophysics said the next step will be to determine the atmosphere of K2-18b, since it's the most promising candidate for sustaining life. And that will rely on the next generation of space telescope, the James Webb, set to launch in 2019.

The planet "is one of our prime targets for observation with the future James Webb Space Telescope," Doyon told CBC News. "It's in the habitable zone, so it's very unique."

As part of the agreement for Canada's contribution to the telescope, the instrument team is guaranteed 450 hours of observation time. 

The Canadian-made Near-InfraRed Imager and Slitless Spectograph (NIRISS) is specifically designed to probe the atmospheres of exoplanets, and Doyon said that K2-18 is at the top of the list. 

"I can't wait," he said.

With the continual discovery of so many exoplanets, in particular those in habitable zones, Doyon said that he's anticipating future discoveries of worlds that could one day be habitable.

And another instrument that will be contributing to the search will be the SpectroPolarimètre Infra-Rouge (SPIRou) instrument  that will be installed at the Canada-France-Hawaii Telescope later this year.

"It's exciting times," he said. "The next decade will be the golden age of exoplanets."