Astronomers using an innovative method have detected the signal of what could be an extragalactic exoplanet. But confirming its existence will be difficult.
The Milky Way is awash in planets: We’ve found almost 5,000 of them to date, not counting thousands more classified as more nebulous “candidates.” This planetary abundance must exist in other galaxies, too, but the usual techniques can’t reach far enough to tell.
Now astronomers, led by Rosanne Di Stefano (Center for Astrophysics, Harvard & Smithsonian) have found a possible extragalactic exoplanet, or as they call it, extroplanet, with results published in Nature Astronomy. But even if it’s real, it’s residing in a pretty inhospitable environment: around a black hole. Specifically, the candidate planet orbits an X-ray-emitting binary system (XRB), where a black hole or neutron star siphons material off a companion star.
That may sound like a surprising place to look for other worlds, but then again, the first exoplanets ever found orbited a compact object (a pulsar) rather than an ordinary star.
Key to the method is the relative size of the emission: As stellar material spirals into the black hole, the stuff nearest the maw — a relatively small region — heats up and emits X-rays. A planet passing in front of this region could block most or all of its radiation, for a period of minutes for an Earth-size planet, up to hours for a Jupiter-size world.
Di Stefano and collaborators set out looking for such a signal, surveying XRBs across three nearby spiral galaxies: two face-on spirals, M51 and M101 (the Whirlpool and the Pinwheel, respectively) and one edge-on, M104 (the Sombrero Galaxy).
The hunt struck gold in the Whirlpool Galaxy, which lies 28 million light-years away. There, the bright XRB M51-ULS-1 system hosts what is likely a black hole feeding off a massive, bright-blue star. The spiraling stellar material makes up an X-ray-emitting region some 50,000 kilometers (30,000 miles) across. Then the astronomers watched the brightness suddenly drop by a factor of 10, staying dim for three hours before returning to baseline.
There’s no reason that the blockage had to be due to a planet, and the team considered other options. But most other possible causes of such a dimming, such as the passage of a gas cloud or an interruption of the black hole’s meal, would cause some energies to dip first or more than others. In contrast, M51-ULS-1 dimmed equally across the board. The passing object couldn’t have been a white dwarf, either, whose extreme gravity would have bent light, magnifying it rather than blocking it.
Di Stefano’s team ran combinations of sizes and distances to see what would match the dip they observed, concluding, “There is a significant probability that the transiting object has a size smaller than Jupiter.”
“I can hardly express how pleased and moved I feel to have discovered this candidate planet in another galaxy,” Di Stefano says. “As researchers, we are always trying to learn more, to discover things that would not otherwise have been found.”
But it’s worth noting that there’s still a chance this object is in the range of brown dwarfs or even low-mass stars. Scott Wolk (also at Center for Astrophysics), who was not involved in the study, notes that many massive star systems have not just two but three or four stars. So it’s not out of the question that the eclipser was a low-mass star in a far-out orbit.
“For this to actually have been a planet would be incredibly lucky,” Wolk adds. The chance of catching a planet in the 2,000 lightcurves the researchers have examined so far is small, he says, on the order of a million to one.
But Di Stefano points out that we don’t actually know the number of XRB systems that have planets. If that number were large, then this result might not be “special” at all. “We need more discoveries to make a real comparison between observations and theory,” she adds. “But I think that is what will happen over the next decade or so.”
While we may never know if a planet caused this particular dip in X-rays, the observations provide a proof of concept that Di Stefano and colleagues hope to use for future discoveries, both in other galaxies and in the Milky Way. “This is how we will learn the most in the near future,” Di Stefano says.
Even if we never make contact with civilizations in other galaxies, by this method we may at least be able to tell if extragalactic planet populations are like our own.