X-rays may have revealed the first planet outside our galaxy

Image of a spiral galaxy.
Enlarge / The site of the X-ray source in the Whirlpool Galaxy.

The Milky Way is full of planets, and it’s very likely that they are plentiful in other galaxies as well.

But there’s a big difference between it being likely that planets exist outside of our galaxy and having evidence that they do. And the methods that have allowed us to spot planets in the Milky Way simply won’t work at such vast distances. But this week, researchers announced that a method they proposed may have turned up the first indication of a planet in another galaxy. The data was sitting in the archives of a couple of X-ray telescopes.

Long-distance eclipse

Almost every planet we know about was identified by one of two methods: either by watching a planet’s gravitational influence on the wavelengths of light produced by a star or by watching the reduction in light as it passes between us and its host star. At the moment, we don’t have hardware with the resolution needed for these methods to work well with other galaxies, which generally appear as collections of stars so dense that distinguishing one star from another is nearly impossible.

In 2018, Nia Imara and Rosanne Di Stefano proposed a variation on existing techniques that might work with distant galaxies. The trick is that it won’t work with visible wavelengths of light.

Consistent X-ray sources in galaxies are relatively rare, meaning that we can point X-ray telescopes at a galaxy and resolve individual sources. Many of these sources are also compact, allowing a planet to obscure them, even if the planet is orbiting at a significant distance. They’re generally composed of the remains of a star, such as a neutron star or black hole, that is powering X-ray emissions by stealing matter off a nearby companion. The process of feeding on this matter is steady enough that these sources tend to emit steadily for long periods of time.

So if the X-ray source were to suddenly wink out and return, Imara and Di Stefano concluded, it would likely be due to an object blocking it along the line of sight from Earth. There are a number of potential objects that could cause this effect, including the star it is drawing matter from. Or it could be an exoplanet.

From hypothesis to data

A few years later, Imara and Di Stefano are back as part of a larger team suggesting that this method seems to work. The data comes from observations of the galaxy M51, also known as the Whirlpool Galaxy. One of the brightest X-ray sources in that galaxy, called M51-ULS-1, is exactly the type of X-ray-emitting binary system that the initial proposal had in mind. It’s composed of an unidentified compact object that appears to be orbiting close to a blue supergiant star. That supergiant appears to be losing matter to the compact object in a way that powers a steady stream of X-rays.

Back in 2012, M51-ULS-1 was in the field of view of the Chandra X-ray Observatory when it suddenly went quiet. Before and after the event, Chandra had been detecting an average of about 15 photons per thousand seconds coming from M51-ULS-1. Then there was a sudden decline and, for over a half-hour, absolutely no photons were detected. About a half-hour later, things were indistinguishable from how they’d been prior to the dip.

The source's X-ray emissions went from a steady stream to zero and then back again.
Enlarge / The source’s X-ray emissions went from a steady stream to zero and then back again.
Di Stefano et. al.

There’s often a lot of variability in X-ray sources, since the inflowing material that powers them can vary and even obscure the source of the X-rays. But those events don’t look like what the researchers are seeing. If an X-ray source goes quiet (or comes back on), it usually occurs very gradually, and intervening matter will tend to block some wavelengths more efficiently than others, leading to a change in the “color” of the light without eliminating the light entirely.

Therefore, the object was most likely a body that passed between Earth and the X-ray source. If it were a planet orbiting a nearby star, then watching to see if it passes in front again would make sense. But that’s not a viable solution here, since any planet is likely to be a long distance from the compact object, which was probably formed by the explosion of a massive star.

To get a sense of what the researchers might be looking at, the team tested a variety of models that varied the orbit and size of the object to see which ones could produce the sort of X-ray dropout seen here. These models suggest that the most probable cause is an object that’s roughly the size of Saturn. That makes it too small to be a star or brown dwarf. Dwarfs could potentially be within the appropriate size range but are massive enough to cause gravitational lensing effects, which weren’t apparent here.

The challenge with this result is that Saturn-sized objects are typically gas giants, and the environment near the X-ray source is probably sufficient to boil away a planet’s atmosphere. So it’s fair to say that even the best solution is probably not ideal at this point.

To the archives!

Overall, the models suggest that whatever might be orbiting there, it’s likely to be on the order of tens of astronomical units from the binary system that produces the X-rays. (For reference, the Earth is located one AU from the Sun). That makes follow-up observations a problem. Neptune is about 30 AU out, and it takes 165 years to complete an orbit. Unless a lot of other planets are in similar orbits, we won’t see another event like this in our lifetimes.

Our best chance to get a better understanding of what we’ve seen is to look for another similar event. And here, the odds are considerably better. Both the Chandra and XMM-Newton X-ray telescopes have been in orbit for over 20 years, and there are extensive archives of data from past observations. Many involved staring at a single location long enough to pick up something similar, and the researchers involved in the new work suggest that most scientists haven’t been interested in short-term fluctuations like this.

For the current work, the team just looked for dips in three galaxies where someone else had processed the data into light curves that allowed an algorithm to search for dips in activity. With a potential success, the researchers are likely to expand their search considerably.

Nature Astronomy, 2021. DOI: 10.1038/s41550-021-01495-w  (About DOIs).

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