Frozen super-Earth exoplanet found in second closest star system

Martin Kornmesser  ESO

Martin Kornmesser ESO

Barnard's Star is the fourth-closest star to our sun.

Yet much about the planet around Barnard's Star remains uncertain.

An global team of astronomers was excited to announce the discovery of a super-Earth - 3.2 times the size of our own - orbiting a star just six light years away called Barnard's star. The results of the study are published in the journal Nature. Astronomers now say it's also home to a frozen exoplanet at least three times as massive as Earth, making it a super-Earth.

The Super-earth, which orbits Barnard's Star in about 233 days, is inhospitable to humans.

The red dwarf star itself emits only about 0.4% of our sun's radiance, so the planet receives about 2% of the intensity that Earth receives from its sun. The "planets", however, ended up being nothing more than an instrumentation problem with the Sproul Observatory in Swarthmore, Pennsylvania, whose director, Peter van de Kamp, first claimed the existence of planets around the star. If a planet produced these changes, then they would appear regularly with each of the planet's orbits. Two years ago, astronomers announced the discovery of a roughly Earth-sized planet circling Proxima Centauri, part of the Alpha Centauri system, in an orbit that might enable liquid water to exist on its surface, raising the possibility that it could harbour alien life.

An artist's conception shows what the surface of the reported planet known as Barnard's Star b might look like.

"After a very careful analysis, we are over 99 percent confident that the planet is there, since this is the model that best fits our observations", said Ignasi Ribas, astronomer at the Institute of Space Sciences in Spain.

Instead, the team used the radial velocity method. With the Doppler effect, as a planet orbits a star, the planet's gravitational pull causes its star to wobble a little bit. One thing we have learned through the exoplanet era is that, where one planet lurks, more are sure to follow.

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Professor Carole Haswell, head of astronomy at the Open University and a member of the worldwide team that announced the discovery in the journal Nature, said: "While the starlight from Barnard's Star is too feeble for Barnard's Star b to have liquid water on its surface, Barnard's Star b probably has a similar temperature to Jupiter's moon Europa".

Yet much about the planet around Barnard's Star remains uncertain. "But in the United States, they are also developing WFirst - a small telescope that's also used for cosmology", said Dr Anglada Escudé. "[The candidate] is very strong in terms of the statistical significance". At the time, the case for a planetary object (instead of some kind of activity intrinsic to the star) wasn't strong enough to stand on its own. It was discovered as part of a project to find rocky planets around red dwarfs and the instruments used to do this-including the CARMENES (Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs)-are specially created to do this.

Graphic representation of the relative distances to the nearest stars from the sun.

"This discovery means a boost to continue searching for exoplanets around our closest stellar neighbors, in hopes that eventually we will come upon one that has the right conditions to host life", said co-author Cristina Rodríguez-López, researcher at the Instituto de Astrofísica de Andalucía (IAA, CSIC).

Because Barnard's Star is so dim, the planet's long orbital period puts it at the "snow line", where sunlight is so faint that its surface is perpetually frozen.

That's what gives the length of the planet's year.

In other words, the most massive planet in a given system should form just beyond the ice line.

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