Astronomers find evidence of an extragalactic exoplanet

Astronomers find evidence of an extragalactic exoplanet

Since the finding of the first exoplanet in 1992, astronomers have discovered countless others. They estimate that the Milky Way has 40 billion worlds.

So it’s simple to imagine planets in other galaxies, especially ones that appear similar to our own. However, there is a problem in detecting these planets.

Other galaxies are so far away and the stars crammed into such a small region of space, as seen from Earth, that it is hard to identify individual ones let alone the effects of any planets around them. So extragalactic planets have sadly eluded astronomers.

Now, Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics, together with numerous colleagues, claim to have discovered a candidate planet in the M51 Whirlpool Galaxy, which is located 23 million light years from Earth in the constellation Ursa Major. M51-ULS-1b, the alien world, is probably somewhat smaller than Saturn and orbits a binary system at a distance of around ten times Earth’s distance from the Sun.

The observation was possible because of a special set of conditions. The planet’s host binary system is made up of a neutron star or black hole that is quickly devouring a massive neighboring star. The infall of stardust releases enormous amounts of energy, making this system one of the strongest X-ray sources in the Whirlpool Galaxy. Indeed, its X-ray luminosity is almost a million times brighter than the Sun’s total output at all wavelengths.

X-Ray images

However, the source of these X-rays, a black hole or neutron star, is extremely small. That means a Saturn-sized planet orbiting a billion kilometers away can completely eclipse the X-ray source, should it pass directly in front in the line of sight with Earth.

That appears to have occurred on September 20, 2012. The orbiting Chandra X-ray Observatory happened to be watching at the moment. The X-ray source dimmed to nothing and then reappeared, with the entire transit lasting around 3 hours.

Nobody noticed at the time because the Chandra data sets were not being checked for such short variations. But when Di Stefano and his colleagues examined closely, they saw the telltale signs.

There are several reasons why an X-ray source may dim in this way. The presence of another tiny star, such as a white dwarf, that eclipses the X-ray source is one example. M51-ULS-1b cannot be a white dwarf or other form of star, according to the study, because the binary system is too young for such an object to have evolved nearby.

Another explanation is natural variation, which maybe caused by an interruption in the material falling into the black hole or neutron star. According to Di Stefano and colleagues, in these cases, the brightness varies in a predictable way, with higher energy light frequencies changing faster than lower energy ones and switching back on in a different way.

Transit time

In this example, however, all of the light frequencies dimmed and reappeared at the same moment, suggesting an eclipse. “It is approximately symmetric, and has a shape typical of transits in which the source and transiting object have comparable size,” they say.

Now that the first planet candidate in another galaxy has been found, Di Stefano and colleagues predict that others will be discovered soon. The team scoured just a portion of the X-ray data from Chandra to find this new planet candidate.

There’s a lot more where that came from. “The archives contain enough data to conduct surveys comparable to ours more than ten times over,” say the team. “We therefore anticipate the discovery of more than a dozen additional extragalactic candidate planets in wide orbits.” And more data is being gathered all the time.

M51-ULS-1b may be the first candidate planet identified in another galaxy, but it is far from the last. Keep an eye on this place.

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Max Burkett
Max Burkett
7 months ago


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