Young Planets Around Old Stars? Astronomers Believe Unseen Planets Exist in Empty Space at Center of Dust Disk

ATLANTA—Astronomers using the Georgia State University Center for High Angular Resolution Astronomy (CHARA) Array telescopes have mapped the orbit of a pair of distant old stars that is surrounded by a vast disk of gas and dust. The disk has a huge hole in the middle that would reach the orbit of Neptune if placed in our solar system.

Astronomers wondered if the central hole in the disk was caused by tidal pulls from the central pair of stars, but the new study shows that the pull of the stars is much too weak to explain the hole. Could recently created planets have formed in the disk and swept away the gas?

This fascinating hypothesis is explored by an international team of scientists including Narsireddy Anugu, an optical systems scientist at Georgia State, in a new study published in The Astrophysical Journal.

“Our colleagues have been analyzing the orbit of AC Herculis for several years," said Gail Schaefer, director of the CHARA Array. “Using the CHARA Array, they resolved the separation between the two stars and mapped their motion over time to develop this compelling hypothesis."

At the end of their lives, stars similar to the Sun swell to become giants and eject material into space before ending up as small and faint white dwarf stars. When such a star has a close stellar partner (in a binary system), the material outflow is channeled by the companion star into a large and stable disk of dust and gas rotating around both stars.

The surprising fact is that about 10 percent of these disks show a large central cavity. Such “circumbinary” disks resemble the so-called “transition disks” seen around young stars, where the inner void of the disk is strongly linked to the presence or formation of giant planets.

Could the circumbinary disks around binary stars be the birthplace of a second generation of planets around very old stars?

Anugu and his colleagues focused on an old binary system known as AC Herculis in the Milky Way Galaxy about 4,600 light-years from Earth. At this distance, the two stars of AC Herculis appear too close together to see as separate objects using conventional telescopes. Instead, the scientists used the powerful CHARA Array telescopes that comprise the largest long-baseline optical interferometer in the world.

The CHARA Array uses six telescopes spread across the top of Mount Wilson in California to act like one enormous telescope that is capable of measuring tiny details in stars. In this case, the scientists found that the stars appear to be separated by a distance similar to the spacing between car headlights as seen from halfway to the Moon.

The astronomers monitored the pair of stars over its three-year orbital period and determined their stellar masses from the orbital motion. They found that any tidal forces from the stars would only disrupt gas and dust in the surrounding disk out to a distance that is at least three times smaller than the gap actually observed.

“These explanations cannot account for the huge hole in the disk, and we suspect that unseen planets exist in the gap that are sweeping out the central region,” Anugu said.

Astronomers think that when disks form around old binaries, the dense conditions may be ideal to form new planets. These planets would accumulate gas from the disk to grow to sizes like that of Jupiter in our solar system. These young planets would witness the fading stages of the old stars around which they orbit.

The CHARA Array, which is operated by Georgia State’s Department of Physics and Astronomy, plays a key role in teaching scientists and students at the university and other institutions worldwide. For more information, visit their website.

Image credit: Dr Mark A. Garlick / markgarlick.com