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Astronomers Find Evidence of an Extragalactic Exoplanet

Vortex Galaxy (M51) Image: NASA, ESA, S. Beckwith (STScI) and Hubble Heritage Team (STScI / AURA)

The Milky Way is full of planets. Now astronomers may have found the first planet candidate in another galaxy.

Since the first exoplanet was discovered in 1992, astronomers have found thousands of other exoplanets . As a matter of fact, it is estimated that our galaxy Milky Way is home to 40 billion worlds .

So it is reasonable to think that there must be planets in other galaxies, especially those similar to ours. But a problem arises when it comes to detecting these planets.

Other galaxies are so distant, and the stars of these galaxies, as seen from Earth, are squeezed into such a small region of space that it is difficult to spot individual stars, let alone show the effect of any planet. For this reason, extragalactic planets are sadly ignored by astronomers.

Now, Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics and several colleagues say they have found a planet candidate in the M51 Vortex Galaxy, about 23 million light-years from Earth, near the Big Dipper Constellation. Called M51-ULS-1b, this distant earth is probably slightly smaller than Saturn and orbits a binary system around its star, ten times the distance from the Sun to Earth.

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Observation was possible due to a particular set of circumstances. The planet’s host binary system consists of a neutron star or black hole that swallows the next star at great speed. The collapse of stardust releases large amounts of energy, making this system one of the brightest X-ray sources in the entire Vortex Galaxy. Essentially, the brightness of this system X-ray is roughly one million times the radiation emitted by the Sun at all wavelengths.

X-Ray Images

However, this is black hole or neutron star , the X-ray source is very small. This means that a planet the size of Saturn a billion kilometers away can only completely obscure the X-ray source if it crosses its line of sight with Earth.

Exactly this seems to happen on September 20, 2012. Luckily, the orbiting Chandra X-ray Observatory was watching that direction at that time. The X-ray source faded and then reappeared, while the entire transition took about 3 hours.

At the time, nobody noticed that such brief changes were not sought in datasets from Chandra. But when Di Stefano and his colleagues looked, the expected signs were clearly visible.

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There are several reasons why an X-ray source may darken in this way. One of them is the presence of another small star like a white dwarf that covers the X-ray source. The team says M51-ULS-1b cannot be a white dwarf or any other type of star because it is too young for such an object to form in the vicinity of the binary system.

Another possible explanation is natural change, perhaps due to interruption of material falling into the black hole or neutron star. Di Stefano and colleagues say that in these situations the luminosity differs in a characteristic way, with high-energy light frequencies changing faster than low-energy light frequencies, and reverting in a different way.

Transition Time

However, in this case, all light frequencies dimmed and flashed at the same time. This suggests an eclipse. Experts say, “The eclipse is about symmetrical, and has a typical form of involvement in which the source and the transitional object have comparable size.”

Di Stefano and his colleagues say that now that the first planet candidate in another galaxy has emerged, others will be found quickly. The team looked at only some of the X-ray data from Chandra to find this new planet candidate.

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There is much more where this data comes from. “The archives contain enough data to run studies comparable to ours,” the team says. “We therefore expect more than a dozen additional, galactic planet candidates to be discovered in large orbits.” And more data is collected all the time.

So while M51-ULS-1b is the first planetary candidate discovered in another galaxy, it is unlikely to be the last.

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