AN EXOPLANET OUTSIDE THE MILKY WAY?
In a galaxy far, far away may lie an exoplanet circling a binary system.
Astronomers think they may have spotted the first extragalactic exoplanet — a planet outside of our own galaxy.
Residing some 28 million light-years away in the Whirlpool Galaxy (M51), the binary system M51-ULS-1 consists of either a neutron star or a black hole paired with a more ordinary companion star 20 times the mass of the Sun.
Evidence for this possible planet came from X-ray data rather than the visual light observations astronomers usually use to detect exoplanets. “We are trying to open up a whole new arena for finding other worlds,” said study lead Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics in a press release.
The new research, published Oct. 25 in Nature Astronomy, examined three galaxies: M51, M101, and M104. The team targeted more than 200 star systems within these galaxies, using the Chandra X-ray Observatory and the European Space Agency’s XMMNewton. Within all those systems, they found only one potential exoplanet.
HUNTING EXOPLANETS
Researchers have mainly used two methods to spot the Milky Way’s more than 4,000 confirmed exoplanets so far. The radial velocity method measures how a star slightly wobbles as an orbiting planet gently tugs on its stellar host. Alternatively, the transit method spots planets that cross in front of their stars, which briefly dims the starlight we detect.
The transit method has been the most useful, spotting planets out to about 3,000 light-years from Earth — well within the boundaries of the Milky Way, which is 100,000 light-years across. To find extragalactic planets, scientists tried searching for transiting planets within X-ray binaries. These systems contain a white dwarf, neutron star, or black hole pulling in material from a companion star. As this material falls into the remnant’s accretion disk, it becomes superheated, producing X-rays.
In these binary systems, the specific area where X-rays are produced within the accretion disk is tiny enough that even a planet can block a significant portion (if not all) of the X-ray light. This makes X-ray transits detectable at much greater distances than visual transits.
In the case of M51-ULS-1, the system
is one of M51’s brightest X-ray binaries. But when astronomers examined Chandra data, they saw that for three hours, the X-rays emanating from the system dropped to 0. According to the researchers, this suggests that a Saturn-sized exoplanet is orbiting the binary system at some 19.2 astronomical units (AU; where 1 AU is the average distance between Earth and the Sun). That’s about twice as far as Saturn is from the Sun.
However, an exoplanet isn’t the only explanation for why the X-ray signal could have been disrupted. X-ray sources can also be obscured by a cloud of dust passing in front of them. The researchers did consider this explanation, too, but ultimately concluded it was less likely than an exoplanet.
Unfortunately, confirming the extragalactic detection will take a long time. With such a wide orbit, the candidate isn’t expected to pass in front of the source again for another 70 years.