Astronomers may have discovered the first planet outside of our galaxy

Astronomers discovered evidence of a possible candidate planet in the M51 ("Whirlpool") Galaxy. This could indicate that the galaxy may be home to the first planet outside the Milky Way. Chandra observed a temporary dimming in X-rays coming from a system in which a large star orbits around a neutron star, or black hole. (See the artist's illustration). This is thought to be a planet passing in front of anX-ray source surrounding the neutron star, or black hole. Credit: NASA/CXC/M. Weiss
For the first time, signs of a planet passing by a star other than the Milky Way Galaxy may have been observed. NASA's Chandra X-ray Observatory has provided a unique opportunity to search for exoplanets from greater distances than before.

The potential exoplanet candidate can be found in the spiral galaxy Messier 51, also known as the Whirlpool Galaxy due to its unique profile.

Exoplanets can be defined as planets that are not part of the Solar System. Astronomers have discovered all known exoplanets or exoplanet candidates within the Milky Way galaxy. They are all located less than 3,000 light years from Earth. A M51 exoplanet would be approximately 28 million light years away. This would mean that it would be thousands of miles farther than the Milky Way.

Harvard & Smithsonian (CfA) in Cambridge, Massachusetts, who led the study, which was published today in Nature Astronomy.

The transits are events that occur when a planet passes in front of a star blocking some of its light. This causes a distinctive dip. Both ground-based and space-based telescopes, such as those used by NASA's Kepler or TESS missions, have searched for dips of optical light, electromagnetic radiation that humans can see. This has allowed the discovery of thousands upon planets.

Instead, Di Stefano and his colleagues looked for dips in the brightness X-rays from Xray bright binaries. These luminous systems often contain a black hole or neutron star that pulls in gas from a nearby companion star. The superheated material close to the black hole or neutron star glows in Xrays.

A planet could pass in front of it, blocking most or all of its Xrays. This makes transits easier because Xrays can disappear completely. This could make exoplanets more easily detected than optical light transit studies currently use. The planet blocks only a small fraction of the star's light, making it possible to detect small decreases in light.

This method was used by the team to find the exoplanet candidate within a binary system named M51-ULS-1. It is located in M51. The binary system is composed of a black hole, or neutron star, orbiting a companion Star with a mass approximately 20 times the Sun. They found an X-ray transit using Chandra data that lasted for three hours. After which, the Xray emission dropped to zero. These and other data led the researchers to conclude that the M51-ULS-1 exoplanet candidate would be approximately the same size as Saturn and orbit the neutron star/black hole at roughly twice the distance from Saturn.

Although this is an intriguing study, it would require more data to confirm the identification as an extragalactic planetet. The problem is that the candidate planet's orbit is so large that it wouldn't cross over with its binary partner for approximately 70 years. This would make it impossible to confirm any observation for many decades.

Nia Imara, co-author at the University of California at Santa Cruz said that it is difficult to confirm that we are seeing a planet. She also stated that she would have to wait for decades before another transit. We wouldn't be able to know when we would look because of uncertainties around the orbital time.

Could the dimming be caused by a cloud composed of dust and gas passing near the X-ray source Researchers consider this unlikely as the events observed in M51-ULS-1 do not support the existence of such a cloud. However, the model of a candidate planet is consistent with the data.

Julia Berndtsson, co-author at Princeton University in New Jersey, said that "We know that we are making an exciting claim so we expect other astronomers to look at it very carefully." "We believe we have a strong argument and this is how science works.

It is likely that a planet in this system has a turbulent past and a turbulent history. A system exoplanet would have to survive the supernova explosion that caused the black hole or neutron star. The future could also be very dangerous. The planet could be hit again by extremely high radiation levels if the supernova-emitting companion star explodes.

Di Stefano and her coworkers searched for X-ray transits within three galaxies that lie beyond the Milky Way. They used Chandra as well as the European Space Agency's XMM–Newton. They searched for 55 systems in M51 and 64 systems in Messier 101 (the “Pinwheel” galaxy), as well as 119 systems within Messier 104 (the “Sombrero” galaxy). The search led to the discovery of one exoplanet candidate.

The authors will search both Chandra's archives and XMM Newton's for exoplanet candidates from other galaxies. There are at least 20 galaxies with substantial Chandra data, including M31 and M33, which are closer to M51. This allows for shorter transits. A second interesting research area is the search for Xray transits in Milky Way Xray sources in order to find new planets in strange environments.

Continue exploring First candidate identified for extragalactic world

Rosanne Di Stefano is a possible candidate planet in an external galaxy, Nature Astronomy (2021). Journal information: Nature Astronomy Rosanne Di Stefano, A possible planet candidate in an external galaxy detected through X-ray transit,(2021). DOI: 10.1038/s41550-021-01495-w