After a blistering search, astronomers may have discovered an exoplanet tens of millions of light-years away.

Astronomers have been scanning the night sky for years, looking for planets outside our solar system. And that is bearing fruit. Thousands of planets have already been discovered around other stars. However, these exoplanets are all less than 3,000 light-years away from Earth. However, researchers are now coming with exciting news. Because they may have discovered a planet outside our Milky Way galaxy for the first time, meaning it’s tens of millions of light-years away.

Messier 51

The researchers tracked down the “exoplanet candidate” using NASA’s Chandra X-ray Observatory. The suspected planet is in the binary star system M51-ULS-1, located in the vortex galaxy Messier 51, an interacting spiral galaxy about 28 million light-years from the Milky Way. This, as you can imagine, is far too far away to observe a planet directly. The researchers therefore looked for small dips in the brightness of the binary star, known as the transit method. Because such decreases can be caused by a planet moving in front of its parent star and temporarily blocking part of the star light.

Transit method
Many telescopes that hunt for exoplanets – think of the now retired Kepler telescope and planet hunter Tess – use the so-called transit method. The brightness of stars is monitored for a long time in the hope that it decreases regularly. Such a regular decrease in the brightness of the starlight may indicate the presence of a planet that occasionally passes in front of the star, blocking some of the starlight. The extent to which the star’s brightness decreases then tells more about the planet’s size; the greater the decrease, the greater the radius of the planet.

In the new study, however, the researchers looked not for dips in optical light, but for decreases in the brightness of X-rays. The studied binary star system M51-ULS-1 consists of a neutron star or black hole, which sucks gas from a companion star. The material near the neutron star or black hole then overheats and glows in X-rays.

X-rays

Because the area that produces bright X-rays is small, a planet passing in front of it could block most or all of the X-rays. And that’s exactly what the researchers discovered. Using the Chandra Observatory, they found an approximately three-hour dip in the X-ray brightness of the binary star in Messier 51. So that could mean that the binary star is accompanied by a planet. Based on various information, the researchers estimate that the suspected planet is about the size of Saturn from our own solar system and orbits a wide orbit — about twice the distance between Saturn and our sun — around the neutron star or black hole.

Composite image of Messier 51. The square indicates the binary star system where the suspected planet may reside. Image: X-ray: NASA/CXC/SAO/R. DiStefano, et al.; Optical: NASA/ESA/STScI/Grendler

These are exciting findings. Because it means that researchers may now have discovered a planet outside our own Milky Way galaxy for the first time. Yet at the same time they are holding a blow. They emphasize that more research is needed to verify results. And that may take some time. Due to the planet’s wide orbit, it could possibly take another 70 years before it again passes in front of its stars and stops X-rays. “To confirm that we see a planet, we probably have to wait decades to see another transit,” said study researcher Via Imara. “And because we don’t know exactly how long it will take for the planet to complete orbit, we don’t know when to watch either.”

Is it a planet?

It is also not yet entirely certain whether the dip in X-ray brightness was actually caused by a planet. Although an alternative explanation such as a dust cloud also seems unlikely. So far, all clues still seem to point in the direction of a planet. “We know we’re making an exciting and bold claim,” said researcher Julia Berndtsson. “We expect other astronomers to look at it very carefully. But we think we have a strong argument.”

If indeed there is a planet in the binary star system, it probably has had quite a tumultuous history. In that case, for example, he probably witnessed the violent supernova explosion that formed the neutron star or black hole. The future also looks turbulent. At some point, the companion star may also explode as a supernova. And that means that the presumed planet will once again be bombarded with extremely high levels of radiation.