Since the first detection of an alien planet back in 1992, thousands of others have been found to date. You can check out the exact number of confirmed exoplanets here. As of writing, NASA’s exoplanet page numbers the confirmed alien planets discovered in the milky way at 4,284.
There are 5,514 exoplanet candidates, all of which are located in as many as 3,179 planetary systems. Needless to say, there are billions of other planets in the Milky Way galaxy alone. Some estimates suggest that there could be over 40 billion planets in our galaxy.
Given this extreme number, it is only logical to conclude that other galaxies across the universe also have billions of planets.
However, given the enormous distances, detecting them with our current technology is quite a challenge.
There are likely billions of galaxies across the universe. The exact number is something that we can’t possibly know, given our current technology.
Other galaxies are so far away that the stars, from our perspective, appear crowded together in a small region of space.
This is why extragalactic planets have long eluded astronomers. Until now, it would seem.
A team of researchers led by Rosanne Di Stefano of the Harvard Smithsonian Center in Massachusetts claims to have found a planet candidate in the M51 galaxy, also known as the Whirlpool galaxy, located around 23 million light-years away, near the constellation Ursa Major.
The alien world, named M51-ULS-1b, is probably slightly smaller than Saturn and orbits a binary system at ten times the Earth-Sun distance.
Discovering this extremely distant world was no easy task for astronomers and was only possible thanks to a special set of conditions.
As revealed by researchers, the extragalactic planet’s host binary system consists of a neutron star or black hole that is devouring a massive nearby star at an enormous rate.
Yielding stardust releases enormous amounts of energy, making this system one of the brightest X-ray sources in the entire Whirlpool galaxy.
In fact, its X-ray luminosity is about a million times brighter than the Sun’s total output at all wavelengths, which is mind.-boggling on its own.
However, the source of these X-rays – the black hole or the neutron star – is diminutive. That means a Saturn-sized planet orbiting a billion kilometers away can completely outshine the X-ray source if it passes directly ahead in between it and our viewpoint from Earth.
NASA’s Chandra X-ray Observatory detected that eclipse by chance on September 20, 2020. The X-ray source dimmed to disappearance and then reappeared, the entire transit lasting approximately 3 hours.
At the time, no one realized why the Chandra data sets were not being searched for such short variations.
But when Di Stefano and his colleagues looked, the telltale signs were clearly seen.
There are several reasons why an X-ray source can be dimmed in this way.
One is the presence of another small star, such as a white dwarf, that overshadows the X-ray source.
The team says that M51-ULS-1b cannot be a white dwarf or another type of star because the binary system is too young for such an object to have evolved there.
Another possible explanation is natural variation, perhaps due to a disruption of material falling into the black hole or neutron star.
Di Stefano and his colleagues say that in these cases, the luminosity changes in a characteristic way, with higher-energy light frequencies changing faster than lower-energy ones and reigniting in a different way. This was not the case, which is why astronomers are confident they’ve actually spotted the very first signal of an extragalactic planet.
Researchers say that all wavelengths of light dimmed and reappeared at the same time, suggesting an eclipse.
“It is roughly symmetrical and has a typical transit shape in which the source and the object in transit are of comparable size,” they explain.
The discovery—if confirmed—would mean that experts have just found a “tool” that can help them search for similar extragalactic planets with our current technology.
The astronomers that made the discovery also scanned a small part of the sky from the Chandra X-ray data. “The archives contain enough data to conduct surveys comparable to ours more than ten times over,” says 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 to be discovered in another galaxy; it is unlikely to be the last the researchers have revealed.
It is noteworthy to mention that the study detailing the discovery of the “first extragalactic planet” has not been peer-reviewed. The study has been published in the preprint server arXiv. You can access the study by clicking here.
Ten things you should know
If confirmed, the discovery of M51-ULS-1b would become a historic moment for astronomy, equivalent to the discovery of the first exoplanet in 1992.
Although exciting, the plant’s existence has still not been confirmed. This doesn’t mean the planet isn’t there; it means that the discovery is so impacting that we need to triple-check the data; everything so far points towards M51-ULS-1b being the first extragalactic world.
Nonetheless, and despite the fact we are still awaiting confirmation, the most likely explanation of the signal spotted by astronomers is a planet.
As revealed in the study published in arXiv, “M51-ULS-1b is the first planet candidate to be found because it produces a full, short-lived eclipse of a bright XRS.”
Astronomers say that M51-ULS-1b has a likely radius that is just smaller than that of Saturn.
The planet is believed to orbit one of the brightest XRSs in the M51, Whirlpool Galaxy.
The host system of the planet consists of a neutron star or a black hole which is feeding on a nearby star at a really fast rate. the infall of stardust releases an incredible amount of energy, which turns the system into one of the brightest sources of X-rays in the entire M51 galaxy; despite the brightness, astronomers say that the black hole—or neutron star is very small. Therefore, a Saturn-size world that orbits the source at a billion kilometers can eclipse the source. This is how astronomers spotted the candidate planet.
This galaxy is located 8.6 Megaparsecs from Earth, or about 23 million light-years away. That’s really, really far away. In comparison, as revealed by LiveScience, it would take 200,000 years for a spaceship traveling at the speed of light to go across the entire Milky galaxy.
Although it is very likely that M51-ULS-1b really is a planet, there are potential natural explanations as well, and one could be an interruption of material falling into the black hole or being sucked towards the neutron star. However, the luminosity would change in specific ways, and this is not the case.
Astronomers say that it is very likely that many other extragalactic planets will be found in the near future as experts go through a plethora of data gathered over the years. “The archives contain enough data to conduct surveys comparable to ours more than ten times over,” astronomers explained.
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Sources and references: Astronomy / arXiv / Astrophysics arXiv