'Fluffy' Earth-mass planet is lightest ever to be weighed

Astronomers have found and measured the mass of the lightest known planet outside the solar system, a "fluffy" celestial body 200 light-years away that weighs as much as Earth but has a much thicker and more gaseous atmosphere.

David Kipping of the Harvard-Smithsonian Center for Astrophysics presented the finding today in Washington, D.C., at the 223rd meeting of the American Astronomical Society. He was the lead author among a team of international astronomers credited with the discovery of the exoplanet, which has been named KOI-314c.

[I]t had exactly the same mass as our home planet of Earth, but it's 60 per cent bigger than the Earth, which means it must have a huge atmosphere sat on top of a rocky core deep down in the middle.- David Kipping, Harvard-Smithsonian Center for Astrophysics

The low-mass planet weighs the same as Earth (1.0 Earth mass units) but is only about 30 per cent more dense than water, which suggests that it's shrouded by an atmosphere composed of hydrogen and helium that could be hundreds of kilometres thick. 

"It's the lightest planet with a measured mass and size ever found," Kipping said in a phone interview from Washington, characterizing KOI-314c as a "fluffy" planet due to its significantly thick atmosphere. 

"The really incredible thing that shocked us is it had exactly the same mass as our home planet of Earth, but it's 60 per cent bigger than the Earth, which means it must have a huge atmosphere sat on top of a rocky core deep down in the middle."

Uninhabitable at 100 C

In kilograms, Kipping said, KOI-314c's mass would be indicated by "a 6 with 24 zeroes after it."

"It's much easier to say it's 1.0 Earth masses," he said.

Life on KOI-314c would be very difficult to sustain, as temperatures there are estimated to be about 100 C.

"We're trying to measure the atmospheres of planets with the ultimate goal that one day we could look at an Earth-like planet and detect oxygen and say therefore it's habitable," Kipping said.

Using data from NASA's Kepler spacecraft, the team was able to determine that KOI-314c orbits a small red dwarf star roughly half the size of our sun and located about 200 light-years away. It circles the star every 23 days.

The team used a new and nontraditional method known as TTV (transit timing variations), which involves measuring the gravitational tug between two planets orbiting the same star — in this case, KOI-314c and a much heavier exoplanet called KOI-314b. By noting the tiny gravitational "wobbles" induced on the planets crossing in front of the host star, the astronomers were able to calculate the masses.

Measuring 'wobbles' to find mass

"The timing of the eclipses varied a little by about half an hour on each side, and by seeing that wobble, we were able to retrieve what the mass of that planet was," Kipping said.

The TTV method only works when more than one planet orbits a star. The conventional method involves measuring tiny wobbles of a star that are induced by a planet's gravity.

"Here, instead of looking at a wobbling star like we would ordinarily, we're measuring the wobbling planet itself. It's a more subtle technique," he said.

Although Kipping said the traditional method could have worked as well, it would have been much more expensive and would have required high-powered telescopes. He said the discovery shows that TTV could be a finer method for detecting low-mass planets outside the solar system. 

"If I wanted to do it the more conventional way, we would have had to wait for maybe a year to collect that data with telescopes," he said. "With the Kepler data, it comes for free. It's a very beautiful thing."

Although KOI-314c is some 200 light-years away from Earth, Kipping said Kepler's observational reach is about 3,000 light-years, which makes the planet "just a stone's throw away by Kepler's standards."