Really far away

Extroplanet: Astronomers may have just found the first planet outside our galaxy

Meet the first extroplanet.

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m51 galaxy and companion
NASA, ESA, S. Beckwith (STScI) and the Hubble Heritage Team (STScI/AURA)

Astronomers may have discovered a planet in a distant galaxy.

Astronomers identified the possible exoplanet, M51-ULS-1b, because its transit blocked X-ray radiation from the remnant of a dead star — either a neutron star or a black hole — near the heart of the spiral galaxy M51, 23 million light years away. The planet is believed to be orbiting in a binary star system.

If confirmed, M51-ULS-1b is the first planet ever found orbiting a star in another galaxy. But its discoverers say more planets may be waiting to be discovered with the same method. The study appeared Monday in Nature Astronomy.

What’s new? — While reviewing archived data from NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton space telescope, Harvard University astronomer Rosanne Di Stefano and her colleagues noticed that the X-ray radiation from a star in spiral galaxy M51 suddenly dimmed, then returned to its normal brightness. They say the dimming is likely the shadow of a planet passing between Earth and the star.

Located at the edge of a young star cluster near the center M51, the star M51-ULS-1 is either a neutron star or a black hole — all that’s left of a once-massive star. It glows so brightly in the X-ray spectrum that astronomers, using orbiting telescopes like Chandra and XMM-Newton, can see it from here in the Milky Way. Its gravity draws a steady stream of material from its nearby partner, a blue supergiant star that appears in Hubble Space Telescope images. The result is a system astronomers call a mass-transfer binary, and it’s about a million times more luminous than our Sun.

The location of putative planet M51-ULS-1b.

X-ray: NASA/CXC/SAO/R. DiStefano, et al.; Optical: NASA/ESA/STScI/Grendler

From 23 million light years away, the planet M51-ULS-1b is visible only as a roughly 3-hour drop in the X-ray radiation coming from the star M51-ULS-1. In archived Chandra observations dating back to 2012, the X-ray “light” coming from M51-ULS-1 dimmed and then went out entirely, before steadily returning to its former brightness. It looked as if something had just passed between Earth and the star, causing a total eclipse.

Dense patches of gas and dust sometimes eclipse distant stars, but the object that briefly blotted out M51-ULS-1 didn’t look like a gas cloud. Instead, it seemed to be symmetrical, with clean, sharp edges — exactly how astronomers would expect a planet to look as it passed in front of its host star.

Based on the timing of transit, and by comparing that data to known objects in our own galaxy, Di Stefano and her colleagues built a computer simulation of the planet’s likely mass, radius, and orbit:

  • It’s probably slightly wider than Saturn, according to Di Stefano and her colleagues
  • It orbits around the binary’s center of mass about 10 to 100 times further than the distance from Earth to the Sun (an astronomical unit, or AU)
  • It orbits the center of mass of both parent stars
  • At that distance, it still gets blasted with as much radiation as a planet orbiting just a few hundredths of an AU from a star like our Sun
  • It isn’t yet known how long its orbit might be

How the planet might have dimmed its home stars.

NASA/CXC/A. Hobart

Here’s the background — Logically, we have every reason to expect that planets form in other galaxies just like they do in our own, but actually finding them has been a challenge.

So far, astronomers have identified around 4,800 planets orbiting other stars in our own Milky Way galaxy. Most of those were discovered using the same transit method Di Stefano and her colleagues used to find M51-ULS-1b. At intergalactic distances, however, it’s nearly impossible to see a single star in enough detail to watch for the faint dip in its brightness that marks a planet’s transit.

In 2018, another team of astronomers found possible evidence of several rogue planets (planets drifting by themselves in interstellar space, not orbiting a star) in a galaxy 3.8 billion light years away. That study measured how gravity affected radiation from near the supermassive black hole at the center of the galaxy.

Its authors noticed that the light bent in ways that suggested the presence of some unseen objects drifting through the galaxy, and they concluded that the otherwise invisible objects were probably rogue planets.

Di Stefano and her colleagues previously suggested that bright X-ray sources — objects that give off a lot of x-ray radiation, such as neutron stars and black holes — could be a good place to look for planets in other galaxies. That’s because the kinds of objects that emit large amounts of x-rays, such as neutron stars and black holes, usually aren’t very wide.

M51-ULS-1 is probably about twice the width of Earth, or around 25,000 kilometers; on the other hand, our Sun is about 1.4 million kilometers wide. A planet passing in front of a small object, like M51-ULS-1, could create a total eclipse, which would be much easier to spot than the small shadow of a planet passing across a much larger star.

And that’s exactly how they spotted M51-ULS-1b, while poring over archived data from Chandra and XMM-Newton.

A diagram breaking down how the discovery was made.


Why it matters — M51-ULS-1b is the first exoplanet candidate astronomers have found orbiting a star in another galaxy; Di Stefano and her colleagues call it an “extroplanet.”

“The search for extroplanets, planets in orbits located outside the Milky Way, has now become a realistic and practical enterprise,” write Di Stefano and her colleagues in their paper. In other words, finding M51-ULS-1b proves that it can be done. It also provides a possible direction for future extroplanet searches.

What’s next? — Di Stefano and her colleagues say there are more planets out there waiting to be found. Archived observations of dozens of galaxies could already contain more transits like the one that pointed to M51-ULS-1b.

“There is no reason to expect that the data sets we employed are extraordinary,” they write, and the current generation of x-ray telescopes, like Chandra and XMM-Newton, are constantly gathering more data on other galaxies for astronomers to work with.

In particular, Di Stefano and her colleagues suggest that two nearby galaxies, M31 and M33 – members of our own galactic neighborhood, the Local Group – could be especially interesting places to look. That’s because they’re close enough that space telescopes can look not only at the brightest X-ray sources, but dimmer ones too.

Those sources will still consist of things like neutron stars or black holes, rather than more familiar main sequence stars like our Sun, but being able to look at slightly dimmer sources expands the number of possible search targets. Additionally, astronomers can see nearby galaxies in enough detail to have a chance of spotting smaller extroplanets, or those orbiting closer to their host stars.

Abstract — Many lines of reasoning suggest that external galaxies should host planetary systems but detecting them by methods typically used in our own Galaxy is not possible. An alternative approach is to study the temporal behavior of X-rays emitted by bright extragalactic X-ray sources, where an orbiting planet would temporarily block the X-rays and cause a brief eclipse. We report on such a potential event in the X-ray binary M51-ULS-1 in the galaxy M51. We examined a range of explanations for the observed X-ray dip, including a variety of transiting objects and enhancements in the density of gas and dust. The latter are ruled out by the absence of changes in X-ray colors, save any with sharp density gradients that cannot be probed with our data. Instead, the data are well fit by a planet transit model in which the eclipser is most likely to be the size of Saturn. We also find that the locations of possible orbits are consistent with the survival of a planet bound to a mass-transfer binary

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