Planets orbit the largest stars

The largest planet to date has been discovered, orbiting two suns

In the constellation Cygnus (Swan) one can find a binary star system that is orbited by a large planet. The binary star system is too faint to see with the naked eye, but a team led by astronomers at NASA Goddard Space Flight Center in Greenbelt, Maryland and San Diego State University (SDSU) in California used the Kepler Space telescope to identify the new planet, which was named Kepler-1647b.

The discovery was announced recently at a meeting of the American Astronomical Society. The research result was published in the Astrophysical Journal with Veselin Kostov, a postdoc at Goddard Space Flight Center, as the lead author.

Kepler-1647 b is 3,700 light-years away and around 4.4 billion years old, which is roughly the same age as our earth. The stars are similar to the sun. One is a little bigger and the other is a little smaller than our sun. The planet's mass and radius is nearly identical to that of Jupiter, making it the largest circumbinary planet to date found in a binary star system.

Planets orbiting two stars are called circumbinary or sometimes tatooine planets, after Luke Skywalker's homeworld in "Star Wars". In the Kepler data, astronomers look for slight drops in the brightness of a star. This could indicate a planet if, in our view, the planet is passing in front of its star and blocking a tiny amount of the star's light.

"But finding circumbinary planets is much more difficult than finding planets around individual stars," said SDSU astronomer William Welsh, one of the co-authors of the article. "The transits do not have a regular interval and can therefore vary in duration and intensity."

The planet's orbital period around its two central stars lasts 1107 days. This is the longest period ever recorded on a transit planet. The planet is also far farther from its stars than any other known circumbinary planet. In doing so, he interrupted the tendency for circumbinary planets to have narrow orbits around their central stars. Interestingly, the orbit of the planet is - despite the enormously long orbital period - in the so-called habitable zone, that is the distance from a star where liquid water would be possible on the surface of a planet.

However, like Jupiter, Kepler-1647b is a gas giant, so this planet is unlikely to be home to life. But if the planet had large moons, they could be suitable for life.

"Leaving habitability aside, Kepler-1647b is important because it could be 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 use advanced computer programs to determine whether it is really a planet. This can be a very exhausting process.

Laurance Doyle, a co-author of the article and an astronomer at the SETI Institute, found a transit in 2011. But more data and multi-year analysis are needed to confirm the transit and that it is indeed caused by a circumbinary planet. A network of amateur astronomers called the Kilodegree Extremely Little Telescope (KELT) is a follow-up network that can help researchers determine the planet's mass through additional observations. KELT consists of two robotic telescopes, KELT-North in Arizona, USA, and KELT-South near Sutherland in South Africa.

June 15, 2016 / SP
Kuffner Observatory Association