Astronomers have found signs of a planet orbiting a star outside our galaxy, the Milky Way, for the first time.
The team found evidence of a possible planet in the spiral galaxy Messier 51 (M51), which is located around 28 million light-years from Earth. The galaxy is also known as the Whirlpool Galaxy because of its distinctive appearance.
They calculated that the exoplanet could be approximately the size of Saturn, the solar system’s second-largest planet after Jupiter, and that it’s orbiting a star and a partner that is either a neutron star or a black hole.
Until now, all the planets found outside the solar system, or exoplanets, have still been located within the confines of the Milky Way. The majority of these planets have also been located within 3,000 light-years of the Earth.
That means that if this is indeed a planet in M51, it is thousands of times more distant than even the furthest previously detected exoplanet.
if confirmed, the planet will join the catalog of over 4,000 of those objects detected thus far, but it will also become the first object in a new catalog of extragalactic planets.
The astronomers who discovered this extragalactic world did so using NASA‘s Chandra X-ray Observatory, searching nearby galaxies in the X-ray region of the electromagnetic spectrum. It is the reliance on X-rays that makes the detection of more distant worlds such as this one, possible.
While even stars in neighboring galaxies can be difficult to distinguish in visible light and other regions of the electromagnetic spectrum, because stars shine less brightly in the X-rays, using these emissions can make distinguishing individual objects much easier.
“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,” researcher from the Center for Astrophysics | Harvard & Smithsonian (CfA), Cambridge, Massachusetts, Rosanne Di Stefano, said in a NASA press release.
Di Stefano was the lead researcher on the study, published in the journal Nature Astronomy.
In order to spot this planet, Di Stefano and the team adopted a technique that has been used with great success to detect exoplanets outside the solar system, but within our galaxy.
As exoplanets pass the face of their parent star, they block the light from that star and cause tiny, but detectable, dips in the star’s light output. This happens for light across the entire electromagnetic spectrum including X-rays.
This transit method of exoplanet detection has been used by astronomers with both ground-based and space-based telescopes – like those carried by NASA’s Kepler and Transiting Exoplanet Survey Satellite (TESS) missions—to discover thousands of exoplanets.
To use the transit method to detect dips in X-ray emissions caused by transiting exoplanets, the team had to use stars that are associated with strong output in this region of the electromagnetic spectrum.
Places to Search for Planets
For this, the team used X-ray binaries, systems of two orbiting stars one of which is a compact stellar remnant, a neutron star or a black hole, that is feeding on material from its companion. When this material falls to the compact object surface, the intense gravitational force heats it tremendously and causes it to release high-energy X-ray emissions.
“X-ray binaries may be ideal places to search for planets, because, although they are a million times brighter than our Sun, the X-rays come from a very small region,” Di Stefano said. “In fact, the source that we studied is smaller than Jupiter, so a transiting planet could completely block the light from the X-ray binary.”
Searching for dips in these X-ray emissions that could be caused by an orbiting star, Di Stefano and the team spotted such a signature around the X-ray binary – M51-ULS-1 – in the galaxy M51.
Studying the signal in detail they discovered the emission of X-rays dropped to zero as something transited the binary. This last for three hours then the X-ray transmission resumed.
After eliminating other possibilities for this dip in X-ray emissions, such as dust or another object, the team concluded that they are caused by a transiting dark planet about the size of gas giant Saturn. The extragalactic planet is orbiting the X-ray binary data at a distance of twice the orbit of Saturn around the sun.
The researchers believe that given the orbit of this possible planet, it won’t transit its parent star and compact stellar companion again blocking X-rays for another 70 years.
“Unfortunately, to confirm that we’re seeing a planet we would likely have to wait decades to see another transit,” said paper co-author and University of California at Santa Cruz researcher Nia Imara. “And because of the uncertainties about how long it takes to orbit, we wouldn’t know exactly when to look.”
If this is a planet in this system, the authors point out that it must have survived the tremendous supernova explosion that created the compact stellar object in the X-ray binary.
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