Not that long ago,, astronomers weren’t sure that exoplanets even existed. Now we know that there are thousands of them and that most stars probably harbour exoplanets. There could be hundreds of billions of exoplanets in the Milky Way, by some estimates. So there’s no reason to think that stars in other galaxies don’t host planets.
But to find one of those planets in another galaxy? That is a significant scientific achievement.
Astronomers find most exoplanets in our galaxy with the transit method. When a planet passes between us and its star, the star’s light dips a tiny amount as the planet blocks out some of the light. Measuring that slight dip is very difficult, but that’s what planet-hunters like NASA’s Transiting Exoplanet Survey Satellite (TESS) do. That method won’t work in another galaxy. It’s difficult to even discern individual stars in other galaxies, let alone detect the minuscule light blockage when a potential exoplanet transits in front of its star.
But TESS observes in visible light and near-ultraviolet light. What if observing a different part of the spectrum allowed astronomers to see individual stars in another galaxy and even planets orbiting those stars?
A team of astronomers have used data from the ESA’s XMM-Newton spacecraft, which observes x-rays, to discern individual stars in another galaxy. There are fewer bright objects in x-rays than in visible light, so identifying the sources of x-rays isn’t as challenging as in visible light.
The team has published a paper in the journal Nature Astronomy titled “A possible planet candidate in an external galaxy detected through X-ray transit.” The lead author is Rosanne Di Stefano from the Center for Astrophysics, Harvard & Smithsonian. In their paper, the researchers present evidence of a Saturn-sized planet orbiting a star in the Whirlpool Galaxy.
The team studied a particular type of star that shines brightly in x-rays. They’re called x-ray binaries because they exist in pairs, and their unique relationship makes them extremely x-ray emissive. An x-ray binary (XRB) consists of a donor star and an accretor. The donor star is usually a fairly typical star, and the accretor is either a stellar-mass black hole or a neutron star.
In an x-ray binary, the more massive accretor draws matter away from the donor star. As that matter falls into the donor, an enormous amount of gravitational potential energy is released and heats the material to millions of degrees. The heated material emits x-rays, and these x-rays are detectable by XMM-Newton.
If a large enough object passes between the x-ray binary and us, we could potentially observe a dip in x-rays, the same way TESS observes dips in visible light.
“X-ray binaries may be ideal places to search for planets because, although they are a million times brighter than our Sun, the X-rays come from a very small region. In fact, the source that we studied is smaller than Jupiter, so a transiting planet could completely block the light from the X-ray binary,” explains first author Rosanne Di Stefano in a press release.
Along with data from XMM-Newton, the team used data from NASA’s Chandra X-ray Observatory. Altogether they examined x-ray data from three galaxies for x-ray transits that might indicate the presence of planets. In the Whirlpool Galaxy, they found a transit that completely blocked the x-ray source for a few hours.
When astronomers detect something novel like this, they have to rule out other explanations before concluding it’s an actual planet. X-ray sources can be variable. There are flares and high and low emission states that can last for long periods of time. X-ray sources like M51-ULS-1 can even undergo x-ray off periods, where no there are no x-ray emissions. The team observed one of those periods, but it was observed separately from a transit.
It could have been an object other than a planet, like a brown dwarf or a red dwarf. But the system is too young for those explanations. And the transiting object was too large.
Changes in the density of the gas and dust in the system could have caused it.But the transiting object has a well-defined surface, which a cloud does not have. Even if the planet is a gas giant or a world with an extensive atmosphere, it still would have a well-defined surface. But the researchers make it clear that they can’t completely rule out a gas cloud. “We note, however, that the characteristics of clouds are so broad that the set of possibilities can never be completely explored and ruled out,” they explain.
There are also accretion dips. But dips are different than transits and can take different shapes, while transits do not. “Dips can differ in shape from the eclipses, and in fact, exhibit a wide range of shapes, even in the light curve of a single source. The most obvious difference between a dip and an eclipse is, however, that dips display energy dependence,” the paper states. The paper also explains that a dip changes the spectral nature of the x-ray signal, which provides “…information about the source and the material in its close environment.”
Another possible explanation is changes in the x-rays coming from the accretor itself. But the team eliminated that possibility because the temperature and the light colours never changed. The light was blocked for several hours, but other than that, it was unchanged.
“We first had to make sure that the signal was not caused by anything else,” says Di Stefano. “We did this by an in-depth analysis of the X-ray dip in the Chandra data, analyzing other dips and signals in the XMM data, and also modelling dips caused by other possible events, including a planet.”
They also considered that the donor star itself might be passing in front of the accretor. That did happen, according to the data from XMM-Newton. But that was separate from the other transit. The donor star transit caused a much more prolonged blackout.
In their paper, the authors wrote, “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 did computer simulations to see whether the dip has the characteristics of a planet transiting, and we find that it fits perfectly. We are pretty confident that this is not anything else and that we have found our first planet candidate outside of the Milky Way,” said Di Stefano.
So if there is a planet there, what are its properties? That’s difficult to say with certainty, but the team of researchers was willing to speculate from their data.
It’s about the size of Saturn, and it orbits the binary star at a great distance: it’s tens of times more distant from the binary than the Earth is from the Sun. It also takes about 70 years to complete one orbit. As for habitability? No way. The binary star bombards the planet with extreme amounts of radiation.
The 70-year orbit is a limitation of this study. The long orbital period means the data only showed one transit. That’s why the team is careful to call their finding a “planet candidate.” There’s no way to confirm it any time soon by observing more transits. They’ve eliminated the other possible causes, but other researchers may find something else in the data. “We can only say with confidence that it doesn’t fit any of our other explanations,” Di Stefano says.
This isn’t the first candidate planet in another galaxy. Other candidate planets have been found using gravitational lensing. In fact, one planet candidate was found in the Andromeda galaxy. But lensing events are random and don’t repeat, so there’s no way to confirm it like astronomers can with transits, which occur with each orbit of a planet.
There’s already one confirmed extra-galactic planet, and it was also found with x-rays. Astronomers found it in 1992 around the pulsar PSR1257 + 12. In fact, there are two or more planets orbiting that pulsar, according to researchers.
“The first confirmed planet outside of our Solar System was found around a pulsar, an object typically observed in X-rays. I am excited that X-rays now also play important step in the search for planets beyond the border of our galaxy,” said Norbert Schartel, XMM-Newton Project Scientist for ESA.
Are we seeing a new method of finding extragalactic planets coming to fruition?
“This work demonstrates a new method with the potential to discover planets in a wide range of systems hosting XRSs (X-Ray Sources). Because the most luminous XRSs can be detected in external galaxies, the search for extroplanets, planets in orbits located outside the Milky Way, has now become a realistic and practical enterprise,” the authors write in their paper.
“Now that we have this new method for finding possible planet candidates in other galaxies, our hope is that by looking at all the available X-ray data in the archives, we find many more of those. In the future we might even be able to confirm their existence,” said Di Stefano.
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