admin A possible Saturn-sized planet identified in the distant Whirlpool Galaxy could be the first exoplanet to be detected outside the Milky Way.
An exoplanet in the spiral Messier 51 (M51) galaxy – also called the Whirlpool Galaxy because of its distinctive shape – would be about 28m light years away.
Dr Rosanne Di Stefano of the Center for Astrophysics at Harvard and Smithsonian in Cambridge, US, who led the research, said: “Since the 1750s, it has been conjectured that the dim distant nebulas, now called galaxies, are island universes: large, gravitationally-bound stellar populations similar to our home, the Milky Way. Our discovery of the planet candidate … gives us the first peek into external populations of planetary systems, extending the reach of planet searches to distances roughly 10,000 times more distant.”
The discovery opens up a new window to search for exoplanets – planets orbiting stars beyond our Sun – at greater distances than ever before. Although nearly 5,000 exoplanets have been detected so far, all of them are in the Milky Way galaxy – with few further than about 3,000 light years from Earth.
An exoplanet in the spiral Messier 51 (M51) galaxy – also called the Whirlpool Galaxy because of its distinctive shape – would be about 28m light years away.
To find the distant planet hiding in this system, astronomers relied on X-ray data rather than more standard visual observations. “We are trying to open up a whole new arena for finding other worlds by searching for planet candidates at X-ray wavelengths, a strategy that makes it possible to discover them in other galaxies,” said study lead Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics in a press release.
The new research, published in Nature Astronomy, examined three galaxies: M51, M101, and M104. The team targeted more than 200 total star systems within these galaxies using the Chandra X-ray Observatory and the European Space Agency’s XMM-Newton. Within all those systems, they found only one exoplanet.
Hunting exoplanets
Researchers have mainly used two methods to spot the over 4,000 confirmed exoplanets so far. The radial velocity method measures how a star slightly wobbles when an orbiting planet around it gently tugs on its stellar host. Even though stars hold considerably more mᴀss than the planets around them, even a peтιтe world can cause its star to move around a bit, leaving an imprint in the star’s light.
The transit method, on the other hand, takes advantage of a planet crossing in front of its star. This briefly dims the starlight by a detectable amount. Even though planets are much smaller than their stars, researchers can measure these small but recognizable fluctuations in brightness.
Although both the radial velocity and transit methods are clearly effective, they are only useful for finding planets out to about 3,000 light-years from Earth. That’s still well within the boundaries of our Milky Way galaxy, which is about 100,000 light-years across.
So, in order to find this first extragalactic planet, scientists opted to search for pᴀssing planets within X-ray binaries. These systems would contain either a white dwarf, neutron star, or black hole pulling in material from a companion star. As this material falls onto the exotic stellar remnant, it becomes superheated, producing X-rays.
Unlike with optical light transits — where a relatively small planet only blocks a tiny amount of starlight — in such binary systems, the area where X-rays are produced is tiny enough that even a planet can block a significant portion (if not all) of the X-ray light. That means that searching for X-ray transits are detectable at a much greater distances than visual transits.
In the case of the M51-ULS-1 system, the black hole or neutron star is closely orbited by a star some 20 times the mᴀss of the Sun. This makes the system one of M51’s brightest X-ray binaries. By examining Chandra data, researchers saw that for 3 hours, the X-rays typically emanating from the system dropped to zero. According to the researchers, this suggests that a Saturn-sized exoplanet is orbiting the compact object 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.
Of course, 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, say, a cloud of dust pᴀssing in front of it. The researchers did consider this explanation, too, but they 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 won’t pᴀss in front of the source again for another 70 years.
If M51-ULS-1 is a planet, however, the Saturn-sized object has a rather tumultuous history.
The presence of a neutron star or black hole means that once upon a time, the system was home to not just the current companion star, but also another dying star. This doomed star would have burned through all of its fuel before erupting as a supernova, bathing any planets nearby with intense radiation.
If a planet really does exist in this system, it is likely to have experienced a violent past. Any exoplanet in the system would have had to survive the cataclysmic supernova explosion that created the neutron star or black hole from a previously existing star. The future may also be dangerous, as at some point the companion star could also explode and blast the planet with extremely high levels of radiation.
Searching for X-ray transits in Milky Way, and other extragalactic X-ray sources, could lead to the discovery of other exoplanet candidates in unusual environments.
