Astronomers have discovered two planets with a mass a few times greater than Earth’s that revolve around a nearby star. Unlike many others that host planetary systems, this one is quite inactive: it does not emit flares of energy that harm the chances of life on the planets.

“It is the best among stars very close to the Sun to know if those planets have an atmosphere and life,” says Sandra Jeffers, an astronomer at the University of Göttingen in Germany, who led the team that discovered them. The finding was published June 25 in Science.

The star, called GJ 887, is just 3.3 parsecs from Earth (10.7 light years), in the constellation Piscis Austrinus. It is the brightest red dwarf star, as seen from Earth.

Red dwarfs are smaller and cooler than the Sun; many have planets that orbit around them. But most are highly active, with a magnetic energy that stirs their surface and releases deluges of charged particles into space during eruptions known as stellar flares. Many famous planetary systems orbit around red dwarf stars, such as Proxima Centauri, the closest star to Earth, and TRAPPIST-1, which have seven worlds the same size as Earth. Astronomers say that the planets in these systems could not sustain life, as their stars constantly bombard them with powerful radiation.

By contrast, the planets in the newly discovered system could survive without much damage. “GJ 887 is interested in the stillness of the central star,” says Jeffers. “It is what is exceptional about it.” His team used several methods to measure the activity of GJ 887. The Exoplanet in Transit Survey Satellite (TESS), a space telescope, observed the star for several weeks; several groups of amateur astronomers did so as well. They barely saw any slight oscillation of its brightness; the impression is that it is very stable. They also studied the spectral characteristics of various wavelengths of the star’s light, which could show whether the energy was overflowing. According to that way of measuring it, GJ 887 is not active.

But it may not always have been so quiet, says James Davenport, an astronomer at the University of Washington in Seattle. The planets have been spinning around GJ 887 for billions of years; the star was perhaps most active in his youth. “So while GJ 887 may be a quiet and cozy place now, it probably had a more dangerous environment when those planets formed,” he says.
If so, the star may have spat out energy-filled flares that would destroy any primitive atmospheres that formed on the planets, leaving them on pure and sterile rock, says Eliza Kempton, an astronomer at the University of Maryland at College Park. “We can’t be sure they have an atmosphere,” he says.

Planets in sight

Jeffers and his collaborators discovered the planets by observing the star every night for three months with a 3.6-meter telescope from the European Southern Observatory in La Silla, Chile. Over time, the movement of the star wobbled slightly as the gravity of the planets pulled it. “We’ve been looking for signs on this star for twenty years,” says Jeffers. “It was such hard work!”

One of the planets has at least 4.2 times the mass of Earth and flies around GJ 887 once every nine days. The other is at least 7.6 times the mass of Earth and describes a turn around the star in 22 days. The two are too close to the star for them to have liquid water on their surface, one of the criteria by which astronomers decide whether a planet is habitable or not. But both are surely rocky, like Earth, and the planet in the 2.2-day orbit may be enveloped in a thick atmosphere, says Jeffers. The wobble of the star also points to a possible third planet, which would be further away than the other two: it would complete an orbit every 51 years and be in the habitable zone of GJ 887.

“What I find admirable is showing again that almost every star in the sky, even the many little ones there are, have planets,” says Davenport. “When we study very close stars, like those, we have the opportunity to truly characterize and know the systems.”

Alexandra Witze / Nature News

Article translated and adapted by Research and Science with permission from Nature Research Group.

Reference: “A multiplanet system of super-Earths orbiting the brightest red dwarf star GJ 887”, by SV Jeffers et al., In Science, volume 368, number 6498, pp. 1477-1481.

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