Astronomers May Have Spotted an Alien World Beyond the Milky Way

'We are trying to open a whole new arena'.
Brad Bergan
The Saturn-sized planet, next to its host star, before a black hole.Center for Astrophysics / YouTube

The universe is vast.

This is why we should expect to find planets not only in our galaxy, but in other ones at great distances. And astronomers have discovered signs of an alien world orbiting a star beyond the confines of the Milky Way, for the first time, according to a recent study published in the journal Nature Astronomy and shared on Chandra's official website.

It resides in the spiral galaxy Messier 51 (M51), also called the Whirlpool Galaxy.

This incredible discovery was made using NASA's Chandra X-ray Observatory, and opens new doors to the search for exoplanets much farther away than ever before.

An alien world in another galaxy could be orbiting a black hole

Exoplanets are worlds residing beyond our solar system, and, until the recent study, astronomers had only discovered exoplanets and possible exoplanets within the Milky Way, nearly all of which are less than roughly 3,000 light-years from our planet. But the one in M51 is roughly 28 million light-years from Earth, thousands of times the distance of other alien worlds discovered in our own galaxy. "We are trying to open up a whole new arena for finding other worlds by searching for planet candidates at X-ray wavelengths, a strategy that makes it possible to discover them in other galaxies," said Lead Author of the study Rosane Di Stefano, of the Center for Astrophysics | Harvard & Smithsonian (CfA), in Cambridge, Massachusetts.

This new find relied on the transit method of observing distant exoplanets, which involves looking for a dip in the star's light created by a planet passing between the star and us. Astronomers employ both space- and ground-based telescopes — like the ones on NASA's TESS and Kepler missions — to "watch" for drops in the intensity of optical light. And this method has enabled the discovery of thousands of planets. Breaking with convention, Di Stefano looked for dips in the brightness of X-rays that come from binary star systems rich in X-ray light. Such luminous systems usually include a neutron star, or a black hole sucking in gas from a nearby companion star.

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The candidate exoplanet from another galaxy might be the size of Saturn

Material near the black hole or neutron star reaches unconscionably superheated temperatures, and begins to glow, in X-rays. But since the region of space generating X-rays is so small, a planet that passes in front of it could block much and possibly most of the X-rays, simplifying the transit detection process. This would enable the detection of alien worlds far more distant than the ones typically detected via conventional optical light transit observations, which require the analysis of tiny variations of light, since the candidate exoplanet can only physically block a small fraction of its host star's light.

The X-ray method is how Di Stefano and the wider research team detected an exoplanet candidate in a binary system dubbed M51-ULS-1, in the M51 galaxy. The system contains either a neutron star or a black hole, orbiting a companion star that's roughly 20 times the mass of the sun. The X-ray transit monitored via Chandra data lasted roughly three hours, during which X-ray emissions dropped sharply, to zero. The researchers consequently suspect an exoplanet might be responsible, roughly the size of Saturn and in orbit of the black hole or neutron star at roughly twice the distance that Saturn orbits our sun. This is an incredible and historic find for modern astronomy, and, although more data is needed to confirm the findings, it marks the beginning of intergalactic exoplanet studies.

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