In a galaxy far, far away, a huge planet may be orbiting a binary star system. If this planet is real, it would be the most distant world we have ever spotted 鈥� the first planet to be found in another galaxy.
A team of researchers led by Rosanne Di Stefano at the Harvard-Smithsonian Center for Astrophysics in Massachusetts spotted this potential planet, which is called M51-ULS-1b. It resides in a galaxy called M51, known as the Whirlpool galaxy, which is 28 million light years away. The researchers found the planet by hunting through data from 2624 observations made by the Chandra X-ray Observatory space telescope.
They scanned the data for signs of transits, which occur when a planet blocks out the light when passing in front of a star or other bright object. To ensure that they only found real transits and not just fluctuations from the bright objects themselves, the researchers only looked for cases in which all of the light was blocked out. They found one possible exoplanet.
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鈥淚t鈥檚 exciting, but not unexpected,鈥� says Angelle Tanner at Mississippi State University. 鈥淭here鈥檚 absolutely no reason to think there wouldn鈥檛 be planets in other galaxies.鈥� This particular planet appears to be orbiting a binary in which a star orbits either a neutron star or a black hole.
The researchers say that the best explanation for the transit is a planet, but it isn鈥檛 entirely certain.
鈥淚t鈥檚 sticky that there鈥檚 only one transit,鈥� says Matthew Kenworthy at Leiden University in the Netherlands. 鈥淭he gold standard is three transits equally spaced from one another, because then you know it repeats,鈥� which indicates that the planet is in orbit, he says.
We have never seen a planet in a system like this, says Tanner, so we don鈥檛 have much in our own galaxy to compare it with. 鈥淚鈥檓 cautiously optimistic, but I would not be surprised if it ended up being something else,鈥� says Tanner. 鈥淚t could be something that just passed in front of this system, never returning again.鈥�
The Chandra measurements indicate that if the planet is real, it is probably a gas giant a bit smaller than Saturn, orbiting tens of astronomical units (AU) from the centre of the binary system. The distance between Earth and the sun is 1 AU, so that puts the planet at least as far from the system it orbits as Saturn is from the sun.
That is potentially a problem for confirming that the planet exists, says Kenworthy. 鈥淚f it鈥檚 more than a few AU out, then it鈥檚 going to be decades before it comes around and causes a transit again,鈥� he says. 鈥淚 can鈥檛 think of a good way how I鈥檇 confirm this.鈥� There have been very few other planet candidates outside our galaxy, and none has ever been confirmed.
If we are able to determine that this planet exists, it will be our first glimpse of a world outside the Milky Way and confirmation that our galaxy isn鈥檛 special in its ability to host planets. 鈥淚t gives us a little bit more of a feeling that maybe we鈥檙e not alone in the universe,鈥� says Tanner.
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