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This is the first extragalactic planet we’ve ever found


We’ve found a lot of planets in recent years. Big and small, far and close, but they all have one thing in common: they’re in our galaxy. Now, a team of researchers from the US and China believe they’ve found the first planet outside of our galaxy, and it’s glorious.


Left: false RGB Chandra/ACIS-S image of the Whirlpool Galaxy, M 51 (total exposure of ≈850 ks). The planet M 51-ULS1 is the orange source at the center of the 60” × 60” dashed white box. Right: HST image of the area defined by the white box in the top image. Image Credits: arXiv:2009.08987 [astro-ph.HE]

 

Galaxies are big. Our galaxy is thought to host more than 100 billion stars, and measure about 100,000 light-years across. In other words, it would take a beam of light 100,000 years to cross the galaxy, and the fastest shuttle we’ve ever built only hit a peak speed of about 3% the speed of light.

 

But even this is just peanuts to space. Our neighboring Andromeda galaxy, for example, is over two times bigger than the Milky Way, and the biggest galaxy we know of (IC 1101) is 50 times the Milky Way’s size and about 2,000 times more massive.

 

The new planet candidate lies a whopping 23 million light years away, in the M51 Whirlpool Galaxy, relatively close to Ursa Major. Normally, it wouldn’t really be possible to identify a planet this far away, but researchers took advantage of a rare set of circumstances.

 


The Whirlpool Galaxy (M51, NGC 5194), a classic spiral galaxy, and its companion NGC 5195. Image credits: NASA / ESA.

 

The object lies in a binary system that has a either black hole or a massive neutron star at the center (we don’t know for sure, but it’s very massive). This object is sucking on a nearby star, and in the process, emitting a huge X-ray signal which caught the attention of astronomers. X-ray signals of this nature are rare on the night sky, so it made for an interesting observation. The X-ray signal also happens to be very small — so small that even a relatively small object passing in front of it would temporarily block it, and this is exactly what researchers have observed.

 

“It is the first candidate for a planet in an external galaxy,” researchers note in the study. If it is confirmed, the planet would be called M 51-ULS1.

 

Simply put, there seems to be a planet passing in between this X-ray source and the Earth, creating an eclipse-type phenomenon. Researchers aren’t exactly sure that it is a planet since it’s too far to observe it directly, but they’ve ruled out all likely possibilities.

 

It will be a while before we can confirm this finding, but for now, it’s safe to say that out of the thousands of planet candidates we’ve found, we also have one outside our galaxy — and that’s pretty awesome in itself.

 

Journal Reference: M51-ULS-1b: The First Candidate for a Planet in an External Galaxy, arXiv:2009.08987 [astro-ph.HE] 

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