European Space Agency (ESA) November 16, 2021

Astronomers detected the temporary dimming of X-rays from a system in which a massive star orbits a neutron star or black hole (as shown). This dimming is interpreted as a planet passing in front of an X-ray source surrounding a neutron star or black hole. Image source: NASA/CXC/M. Weiss

Using ESA's XMM-Newton and NASA's Chandra X-ray space telescope, astronomers have taken an important step in the search for planets outside the Milky Way.

It is difficult to find a planet in another galaxy. Although astronomers know that they should exist, so far no planetary system outside the Milky Way has been confirmed. Because the light from another galaxy is concentrated in a small area of ​​the sky, it is difficult for a telescope to distinguish one star from another, let alone the planets orbiting them. Common techniques for finding exoplanets in our galaxy do not apply to planets outside the galaxy.

This picture shows the vortex galaxy (M51) seen at X-ray wavelengths by the XMM-Newton X-ray Observatory of ESA. Credit: ESA

This is different when studying X-rays in galaxies instead of visible light. Since there are fewer objects that emit light under X-ray light, X-ray telescopes like ESA's XMM can distinguish objects more easily when observing galaxies. Therefore, these objects are easier to identify and study, and it is possible to find a planet around them.

Some of the brightest objects that can be studied in outer galaxies are so-called X-ray binaries. They consist of a very dense celestial body-a neutron star or a black hole-that swallows material from a companion or "donor" star orbiting it. The falling matter is accelerated by the strong gravitational field of a neutron star or black hole and heated to millions of degrees, producing a large amount of bright X-rays. Astronomers predict that, theoretically, planets passing in front of such a source (transit) will block these X-rays, causing the observed X-ray light curve to drop.

"X-ray binary stars may be ideal places to find planets, because although they are a million times brighter than our sun, the X-rays come from a very small area. In fact, the source we are studying is smaller than Jupiter, so a transit Sun planets can completely block the light from X-ray binary stars,” explained Rosanne Di Stefano of the Harvard and Smithsonian Center for Astrophysics in the United States, the author of a new study first published today in Natural Astronomy.

The composite image of M51 with Chandra's X-rays and NASA's Hubble Space Telescope's optical light contains a box that marks the location of possible candidate planets. Image source: X-ray: NASA/CXC/SAO/R. Di Stefano et al.; Optics: NASA/ESA/STScI/Grendler

Rosanne and colleagues searched for such X-ray transits in Chandra and XMM-Newton data of three galaxies, tilted under light that can be explained by planets. They found a very special signal in the spiral galaxy (M51), and they decided to study it in more detail. The drop occurred in the X-ray binary star M51-ULS-1 and completely blocked the signal for several hours before it returned again.

Now, before researchers can even consider choosing an exoplanet, the game of carefully ruling out possible explanations begins. "We must first ensure that the signal is not caused by any other reason," said Rosanne, whose team opposed multiple possibilities in their new publication. "We do this by in-depth analysis of the X-ray inclination angles in the Chandra data, analyzing other inclination angles and signals in the XMM data, and simulating the inclination angles caused by other possible events (including planets)."

Infographic: A planet in another galaxy. Credit: ESA

Is the X-ray inclination caused by small stars such as brown dwarfs or red dwarfs? No, they argued that the system is too young and the transit objects are too big.

Could it be a cloud of gas and dust? The team stated that it is unlikely because the inclination indicates a transit object with a well-defined surface, which is different for passing clouds. Even if the planet has an atmosphere, its surface is still clearer than the clouds.

Can this decrease be explained by the brightness change of the light source itself? The author of the paper is convinced that this is not the case, because although the light from the light source disappears completely for several hours before it returns, the temperature and light color remain the same.

Finally, the team compared this inclination with another light block caused by the "donor" star passing in front of the compact star. This part was observed by XMM-Newton and caused a longer power outage, which is different from the inclination caused by the possible planets.

"We conducted computer simulations to see if the inclination angle has the characteristics of planetary transits, and we found it to be very suitable. We are very confident that this is nothing else, we have found the first candidate planet outside the Milky Way," Rosanne Added.

The team also inferred the characteristics of this planet based on their observations: it will be the size of Saturn, orbiting the binary star system from dozens of times the distance between the Earth and the Sun. It will complete a complete orbit approximately every 70 years and will be bombarded by a large amount of radiation, making it impossible to survive the life on Earth as we know it.

The long orbit of the planetary candidate is also a limitation of this study, because the event cannot be repeated in the short term. This is why the team is still cautious to say that they have found a possible planetary candidate, and the wider community may find other explanations for this, although they have not been found after careful study by the team. "We can only confidently say that it doesn't fit any of our other interpretations," Rosanne clarified.

Nonetheless, this is an exciting step forward in the search for planets outside the Milky Way. Compared with the candidate planets discovered using gravitational lensing, this is the first candidate planet to orbit a known host system. This will also be the first time a planet orbiting an X-ray binary star has been discovered. The existence of these planets is consistent with the fact that planets are found around pulsars (fast-rotating neutron stars), some of which were part of X-ray binaries in the past.

"The first planet we found outside the solar system was found around a pulsar, a pulsar is an object usually observed in X-rays. I am very happy that X-rays are now also playing a role in finding planets beyond the boundaries of the Milky Way. Played an important role,” said Norbert Schattel, a scientist at ESA’s XMM-Newton project.

"Now that we have this new method of finding possible planetary candidates in other galaxies, we hope that by looking at all available X-ray data in the archive, we can find more such data. In the future, we can even confirm them. Existence," Rosanna said.

More information about this discovery:

Reference: Rosanne Di Stefano, Julia Berndtsson, Ryan Urquhart, Roberto Soria, Vinay L. Kashyap, Theron W. Carmichael, and Nia Imara, "Candidates for planets in outer galaxies detected by X-ray transits", 2021 October 25th, Nature Astronomy. DOI: 10.1038/s41550-021-01495-w PDF

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