New Planet Is Largest Discovered orbiting two stars

If you cast your eyes toward the constellation Cygnus, you’ll be looking in the direction of the largest planet yet discovered around a double-star system. It’s too faint to see with the naked eye, but a team led by astronomers from NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and San Diego State University (SDSU) in California, used NASA’s Kepler Space Telescope to identify the new planet, Kepler-1647b.

The discovery was announced today in San Diego at a meeting of the American Astronomical Society. The research has been accepted for publication in the Astrophysical Journal with Veselin Kostov, a NASA Goddard postdoctoral fellow, as lead author.

Not only is it the largest “circumbinary” planet, it also has one of the longest orbits ever recorded for a transiting planet, taking 1,107 days to complete its circuit.

But it isn’t the only body on the move. As Kepler-1647b travels around the system, the two stars are, themselves, in orbit around each other.

“Every 11 days the stars eclipse each other, so it is like a clock,” said Veselin Kostov, lead author of the new research from Nasa’s Goddard Space Flight Center.

For an observer watching from Kepler-1647b, that could lead to an intriguing spectacle.

“Sometimes one will be able to see first the larger star rise or set followed by the smaller one,” said Tobias Cornelius Hinse, a co-author of the research from the Korea Astronomy and Space Science Institute. “But, under special circumstances, one could also imagine [seeing] only one star setting or rising, when the smaller star is hiding behind the larger one during sunrise or sunset.”

“But finding circumbinary planets is much harder than finding planets around single stars,” said SDSU astronomer William Welsh, one of the paper’s coauthors. “The transits are not regularly spaced in time and they can vary in duration and even depth.”

Comparison of illustrations of sizes of Kepler circumbinary planets
Comparison of the relative sizes of several Kepler circumbinary planets. Kepler-1647 b is substantially larger than any of the previously known circumbinary planets.
Credits: Lynette Cook

“It’s a bit curious that this biggest planet took so long to confirm, since it is easier to find big planets than small ones,” said SDSU astronomer Jerome Orosz, a coauthor on the study. “But it is because its orbital period is so long.”

The planet takes 1,107 days – just over three years – to orbit its host stars, the longest period of any confirmed transiting exoplanet found so far. The planet is also much further away from its stars than any other circumbinary planet, breaking with the tendency for circumbinary planets to have close-in orbits. Interestingly, its orbit puts the planet with in the so-called habitable zone–the range of distances from a star where liquid water might pool on the surface of an orbiting planet

Like Jupiter, however, Kepler-1647b is a gas giant, making the planet unlikely to host life. Yet if the planet has large moons, they could potentially be suitable for life.

“Habitability aside, Kepler-1647b is important because it is the tip of the iceberg of a theoretically predicted population of large, long-period circumbinary planets,” said Welsh.

Once a candidate planet is found, researchers employ advanced computer programs to determine if it really is a planet. It can be a grueling process.

Together with ground-based observations from made by both professional and amateur astronomers, the team concluded that the two stars orbit each other every 11 days. Kepler-1647b was found to orbit the two-star system every 1,107 days at 2.7 times the distance of the Earth from our sun. The long orbit of the Kepler-1647b compared to similar previously observed planets, says Hinse, backs up the theory that such planets form at large distances from their stars and then spiral in towards them.

The team also found that Kepler-1647b falls within the “habitable” zone of the double-sun system – the distance from the stars at which it is possible for liquid water to be present. Not that Kepler-1647b is likely to harbour life as we know it – the planet was found to be a gas giant of a similar size to Jupiter.

But Hinse believes there could be other possibilities. “One potential place to have life in this system would be if Kepler-1647b is being orbited by a moon – because that moon would always be in the habitable zone,” he said.

What’s more he adds, his calculations have thrown up another avenue to explore. “There is actually a region between the [two stars] and the planet [Kepler-1647b] where you can have an Earth-like planet on a stable orbit for billions of years,” said Hinse. “We didn’t detect it, but it is possible.”

According to Kostov, the hunt for planets outside our solar system is set to become even more exciting. Next year will see the launch of Nasa’s Transiting Exoplanet Survey Satellite (TESS) – an instrument that will monitor more than 200,000 stars for signs of orbiting planets.

“There are plenty of mysteries,” said Kostov. “We are just touching the tip of the iceberg now.”

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