Planets can form in the galactic center (Near a Black Hole)

Fig : In this artist's conception, a protoplanetary disk of gas and dust (red) is being shredded by the powerful gravitational tides of our galaxy's central black hole. // Credit: David A. Aguilar (CfA)
  
By Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts

  
Published: September 12, 2012
 
At first glance, the center of the Milky Way seems like a very inhospitable place to try to form a planet. Stars crowd each other as they whiz through space like cars on a rush-hour freeway. Supernova explosions blast out shock waves and bathe the region in intense radiation. Powerful gravitational forces from a supermassive black hole twist and warp the fabric of space itself.

Yet new research by astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) shows that planets still can form in this cosmic maelstrom. For proof, they point to the recent discovery of a cloud of hydrogen and helium plunging toward the galactic center. They argue that this cloud represents the shredded remains of a planet-forming disk orbiting an unseen star.

“This unfortunate star got tossed toward the central black hole. Now it’s on the ride of its life, and while it will survive the encounter, its protoplanetary disk won’t be so lucky,” said Ruth Murray-Clay of the CfA.

Last year, a team of astronomers discovered the cloud in question using the Very Large Telescope in Chile. The group speculated that it formed when gas streaming from two nearby stars collided, like windblown sand gathering into a dune.

Murray-Clay and colleague Avi Loeb propose a different explanation. Newborn stars retain a surrounding disk of gas and dust for millions of years. If one such star dived toward our galaxy’s central black hole, radiation and gravitational tides would rip apart its disk in a matter of years.

They also identify the likely source of the stray star — a ring of stars known to orbit the galactic center at a distance of about one-tenth of a light-year. Astronomers have detected dozens of young, bright O-type stars in this ring, which suggests that hundreds of fainter Sun-like stars also exist there. Interactions between the stars could fling one inward along with its accompanying disk.

Although this protoplanetary disk is being destroyed, the stars that remain in the ring can hold onto their disks. Therefore, they may form planets despite their hostile surroundings.

As the star continues its plunge over the next year, more and more of the disk’s outer material will be torn away, leaving only a dense core. The stripped gas will swirl down into the maw of the black hole. Friction will heat it to high enough temperatures that it will glow in X-rays.

“It’s fascinating to think about planets forming so close to a black hole,” said Loeb. “If our civilization inhabited such a planet, we could have tested Einstein’s theory of gravity much better, and we could have harvested clean energy from throwing our waste into the black hole.”

Keck Interferometer Nuller Spots Double Dust Cloud

Credit: NASA/GSFC/Marc Kuchner and Francis Reddy
This graphic compares the inner and outer disk of the 51 Oph system to the location of the planets and asteroid belt of the Solar System.


KAMUELA, Hawaii (Sept. 24, 2009) — Linking the twin, 10-meter telescopes in Hawaii, astronomers at the W. M. Keck Observatory discovered an extended, double-layered dust disk orbiting 51 Ophiuchi, a star that is 410 light-years from Earth. It is the first time the Keck Interferometer Nuller instrument has identified such a compact cloud around a star so far away.

The new data suggest that 51 Ophiuchi is a protoplanetary system with a dust cloud that orbits extremely close to its parent star, said University of Maryland astronomer Christopher Stark, who led the research team.

Keck Observatory operates one of the largest optical interferometers in the United States. The interferometer provides high precision resolution measurements equal to a telescope as large as the distance that separates the telescope’s primary mirrors—85 meters in the case of the Keck twins. In April 2007, the team simultaneously pointed both Keck telescopes at the star 51 Ophiuchi, or 51 Oph, and used the Interferometer’s Nuller, a technique to combine the incoming light in a particular way, to block the unwanted starlight of 51 Oph and measure faint adjacent signals from the dust cloud surrounding the star.

According to the observations, excess material orbited 51 Oph. Stark and his collaborators repeated the nulling measurements at several different wavelengths of light and combined this data with information from other telescopes to determine the shape and orientation of the material as well as the sizes of the dust grains.

The data suggest that two debris disks orbit 51 Oph. The inner disk has larger grains, roughly 10 micrometers or larger in diameter, and extends out to four astronomical units, or AUs, beyond the star. The second disk comprised of mainly 0.1 micrometer grains extends from roughly seven AU to 1200 AU. One AU is the distance between Earth and the Sun or roughly 93 million miles. The new results appear in the Oct. 1 Astrophysical Journal.

If these debris disks orbited the Sun, the inner cloud of larger grains would extend roughly from the position of Mercury’s orbit to just past the edge of the asteroid belt. The outer disk of smaller grains would originate just before Saturn’s orbit and extend to a distance ten times farther than the edge of the Kuiper belt.

51 Oph’s inner, compact dust disk is one of the most compact dust clouds ever detected, and the new Keck Interferometer Nuller observations demonstrate the instrument’s ability to detect dust clouds a hundred times smaller than a conventional telescope can observe, Stark said.

The instrument was also essential to solving the mystery of what made 51 Oph’s dust disk appear so compact while its spectra, or chemical fingerprints, suggested that the dust orbited at much larger distances, added Marc Kuchner, an astronomer at NASA’s Goddard Space Flight Center in Greenbelt, Md. who was part of the research team. The answer was simply that the star had two debris disks.

Because of the power of the Keck Nuller, Stark and his team were able to resolve inner and outer dust disks, which together form 51 Oph’s exozodiacal cloud. In similar star systems, the outer cloud of dust seems to be a distinct outer belt, probably analogous to the Kuiper belt or a second system of asteroids. But 51 Oph appears to be different, Kuchner said. The observations suggest that the star’s outer cloud is comprised of smaller grains and is connected to the inner cloud so that the system has only one underlying belt of asteroids.

This system most likely represents a rare, nearby example of a young planetary system just entering the late stages of planet formation. Terrestrial planets may be forming, although none have been detected within the system yet, Stark said.

His team’s data also indicates that the cloud around 51 Oph is 100,000 times more dense than the dust cloud circling the Solar System. In most planet-forming systems, as asteroid and comet collisions produce dust, the larger grains spiral toward the star while its outward pressure pushes smaller particles to the edge or even out of the system. 51 Ophiuchi, a star 260 times more luminous than the Sun, likely pushes the smaller dust grains from the inner disk to the outer disk, Kuchner explained.

Keck’s Nuller, which was funded by NASA and built by the Jet Propulsion Laboratory in Pasadena, Calif., will be used to help astronomers further understand how and when these asteroid belts form and how dust from the star’s debris disk might interfere with direct imaging of planets orbiting another star, he said.