Astronomers once wondered if there were other planets in the heavens, and we certainly know the answer to that one: a resounding yes. With the help of instruments like the dearly departed Kepler Space Telescope, we’ve discovered thousands of exoplanets, but most of those are within a few thousand light-years of Earth. A new discovery courtesy of NASA’s Chandra X-ray Observatory could point the way to yet another exoplanet, but this one is a bit more distant. If confirmed, it would be the very first exoplanet discovered in another galaxy. Confirmation is not likely unless you’re very patient, though.
The planet in question, if it exists, is in a galaxy known as M51 about 28 million light-years away. You may know it as the Whirlpool Galaxy — even if you don’t know it by name, you’ve probably seen a picture of it (above) as it’s one of the prettiest spiral galaxies visible from Earth. The team, largely from Harvard-Smithsonian Center for Astrophysics, identified a potential planet in a solar system of M51 dubbed M51-ULS-1. The only reason we have any idea of what’s going on in ULS-1 is because it’s so unlike our solar system.
M51-ULS-1 is an X-ray binary, which means it has a sun-like “main sequence” star with a smaller companion in orbit. That companion is either a neutron star or a black hole, and as a result, the system is a powerful X-ray source. Those X-ray emissions appeared in data collected by Chandra, which can be used in a similar fashion to Kepler’s data.
With Kepler and other planet-hunting instruments, astronomers are looking for exoplanet transits. That means the exoplanet passed in front of its star from our perspective here on Earth, causing a small dip in luminance. That kind of signal would be too faint in the visual spectrum to pick up from millions of light-years away, but an X-ray binary’s signature is more intense and compact. Thanks to Chandra, the team was able to identify what looks like a transit in front of the X-ray source, which could be an exoplanet.
With most exoplanet detections, we don’t need to wait long for confirmation. Our methods are better at identifying large exoplanets that orbit close to their stars, but M51-ULS-1 is different. Astronomers estimate because of the suspected orbit, they would need to watch the X-ray source for another 70 years to confirm a transit, but it’s unlikely anyone will still be looking at that point. We will hopefully have much more advanced ways of studying exoplanets by the dawn of the 22nd century. However, it shows this method could be capable of definitively identifying a different exoplanet in another galaxy. It’s probably only a matter of time.
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