It’s a cool planet in a tight spot. A frigid world about 3000 light years away offers the first evidence that rocky planets can form in Earth-like orbits, even when they are jammed into close double-star systems. Although this planet probably cannot support life as we know it, the discovery greatly expands the places we can look for potentially habitable worlds outside our solar system.
In a binary star system the two partners are locked in an orbital embrace. Astronomers have found a handful of planets that orbit both partners in close binary pairs. But no one was convinced that a planet could orbit just one star at a sufficient distance to host life.
“Most stars are part of binary systems, and a significant fraction of these are close binary systems, so if you want to maximise the places you can look for habitable planets, you’re going to want to look at these close binaries as well,” says Scott Gaudi at Ohio State University (OSU) in Columbus.
Gaudi and his team used a technique called gravitational microlensing to study a binary system with two red dwarfs, small stars that are dimmer than the sun. The distance between the stars is about 10 to 15 times that of Earth and the sun. The team found a planet about twice the mass of Earth orbiting just one of the two stars, at about the same distance as we are to our home star.
Solar system snub
Some scientists argue that the planet formation process would get too disrupted if a star has a tight stellar companion, and the further it is from the star, the more difficult it would be for planets to form. “But this discovery argues that yes, indeed, at least in this system of two red dwarfs, you can form planets at these sorts of longer distances,” says Gaudi.
Jean-Philipe Beaulieu at the Paris Institute of Astrophysics in France called the newfound planet an important discovery. “This shows more than ever that our solar system is not the paradigm in our galaxy,” says Beaulieu.
While the planet is most likely rocky based on its mass, its surface is a frigid -213°C. That’s because, at its orbital distance, it does not get enough heat from its dim host star. But the same planet orbiting a sun-like star in a binary system would be in the habitable zone, where conditions could support liquid water and perhaps life as we know it.
The discovery was announced the same week that other scientists sounded a note of caution on exoplanet finds. An analysis led by astronomer Paul Robertson at Pennsylvania State University concluded that Gliese 581 d and g, two of the first potentially habitable worlds ever found, are actually illusions created by sunspots on the parent star.
Gaudi’s co-author Andrew Gould, also at OSU, says his team’s evidence for the existence of their frozen planet is airtight. Microlensing detects planets by watching how their gravity affects the light of a distant background star. As one star passes in front of another, as seen from Earth, light from the background star is gravitationally bent and magnified, projecting images next to the foreground star. But if the foreground star hosts a planet, the world’s gravity can sometimes eliminate one of these images. “The planet isn’t blocking the background star’s light. It’s just that the mathematics of focusing is disrupted by the planet’s gravity,” he says.
“There’s absolutely no doubt that what we’ve detected is a rocky planet in a binary star system,” adds Gould. “Ten thousand years from now, people will go visit this system and find out that it’s exactly what we say it is.”
Beaulieu also expressed confidence in the frozen exoplanet discovery: “In the case of the planet announced by Gould’s team, stellar variability could not mimic the observed signal. This is a very robust detection.”
Journal reference: Science, DOI: 10.1126/science.1251527
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