A newly discovered planet in a binary star system located 3,000 light-years from Earth is expanding astronomers’ notions of where Earth-like and even potentially habitable planets can form, and how to find them.
Although Earth orbits a single star, most sun-like stars are binaries, two stars orbiting each other as a pair. In fact, there are many three-star systems, and even some that harbor up to seven stars. At twice the mass of Earth, the planet orbits one of the stars in the binary system at almost exactly the same distance from which Earth orbits the sun. However, because the planet’s host star is much dimmer than the sun, the planet is much colder than the Earth, in fact, colder than Jupiter’s icy moon Europa.
Four international research teams, led by Professor Andrew Gould of The Ohio State University, published their discovery in the July 4 issue of the journal Science. The study provides the first evidence that terrestrial planets can form in orbits similar to Earth’s, even in a binary star system where the stars are not very far apart. Although this planet itself is too cold to be habitable, the same planet orbiting a sun-like star in such a binary system would be in the so-called habitable zone, the region where conditions might be right for life.
The new find suggests that such worlds may be common and the strategy used to discover the planet could help reveal more exoplanets in the future, researchers say.
Worlds that orbit around twin suns are known as circumbinary planets. The first real-life circumbinary planet ever discovered by astronomers is Kepler-16b, a gas giant found orbiting the star Kepler-16 about 200 light-years from Earth. In another scenario for binary stars, planets can circle one star but not the other. These planets are known either as circumprimary or circumsecondary planets, depending on which star they orbit in the binary system, the brighter or more massive star, referred to as the primary star, or the fainter or less massive one, called the secondary star.
The exoplanet in question has the alphabet soup name of OGLE-2013-BLG-0341LBb. It has about twice the mass of Earth and lies about 3,000 light-years from Earth.
Both stars in the system are red dwarfs that are much colder and dimmer than the sun. The planet’s host star is about 0.11 to 0.14 times the sun’s mass, while its brighter companion is about 0.12 to 0.17 times the sun’s mass. The exoplanet orbits roughly 0.8 AU from its star. In comparison, the planet’s host star lies 10 to 14 AU from its companion, about the same distance between Saturn and the sun.
Although OGLE-2013-BLG-0341LBb is closer to its star than Earth is to the sun, it is much colder because its host star shines 400 times less brightly than the sun. This means the exoplanet does not lie within its star’s habitable zone, the region around a star warm enough for a world to have liquid water on its surface. As such, this exoplanet is much colder than the Earth, with a surface temperature of about minus 350 degrees Fahrenheit (minus 213 degrees Celsius), researchers estimate.
It was a mystery until now whether an Earth-sized planet could form at an Earth-like distance from a star with a companion nearby. Scientists had suggested the close presence of a companion star could disrupt the protoplanetary disk of matter that produces worlds.
“It’s very interesting such planets could form and survive,” study lead author Andrew Gould, an astronomer at The Ohio State University in Columbus said.
“This new finding greatly expands the potential locations to discover habitable planets in the future,” study co-author Scott Gaudi, an astronomer at The Ohio State University, said in a statement.
When the astronomers succeeded in detecting this new planet, they were able to document that it produced two separate signatures, the primary one, which they typically use to detect planets, and a secondary one that had previously been only hypothesized to exist.
The first was a brief dimming of light as the planet’s gravity disrupted one of the magnified images of the source star. But the second effect was an overall distortion of the light signal.