Signs of a planet transiting a star outside of the Milky Way galaxy may have
been detected for the first time. This intriguing result, using NASA's
Chandra X-ray Observatory, opens up a new window to search for exoplanets at
greater distances than ever before.
The possible exoplanet candidate is located in the spiral galaxy Messier 51
(M51), also called the Whirlpool Galaxy because of its distinctive profile.
Exoplanets are defined as planets outside of our Solar System. Until now,
astronomers have found all other known exoplanets and exoplanet candidates
in the Milky Way galaxy, almost all of them less than about 3,000
light-years from Earth. An exoplanet in M51 would be about 28 million
light-years away, meaning it would be thousands of times farther away than
those in the Milky Way.
"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," said Rosanne Di Stefano of
the Center for Astrophysics | Harvard & Smithsonian (CfA) in Cambridge,
Massachusetts, who led the study, which was published today in Nature
Astronomy.
This new result is based on transits, events in which the passage of a
planet in front of a star blocks some of the star's light and produces a
characteristic dip. Astronomers using both ground-based and space-based
telescopes—like those on NASA's Kepler and TESS missions—have searched for
dips in optical light, electromagnetic radiation humans can see, enabling
the discovery of thousands of planets.
Di Stefano and colleagues have instead searched for dips in the brightness
of X-rays received from X-ray bright binaries. These luminous systems
typically contain a neutron star or black hole pulling in gas from a closely
orbiting companion star. The material near the neutron star or black hole
becomes superheated and glows in X-rays.
Because the region producing bright X-rays is small, a planet passing in
front of it could block most or all of the X-rays, making the transit easier
to spot because the X-rays can completely disappear. This could allow
exoplanets to be detected at much greater distances than current optical
light transit studies, which must be able to detect tiny decreases in light
because the planet only blocks a tiny fraction of the star.
The team used this method to detect the exoplanet candidate in a binary
system called M51-ULS-1, located in M51. This binary system contains a black
hole or neutron star orbiting a companion star with a mass about 20 times
that of the Sun. The X-ray transit they found using Chandra data lasted
about three hours, during which the X-ray emission decreased to zero. Based
on this and other information, the researchers estimate the exoplanet
candidate in M51-ULS-1 would be roughly the size of Saturn, and orbit the
neutron star or black hole at about twice the distance of Saturn from the
Sun.
While this is a tantalizing study, more data would be needed to verify the
interpretation as an extragalactic exoplanet. One challenge is that the
planet candidate's large orbit means it would not cross in front of its
binary partner again for about 70 years, thwarting any attempts for a
confirming observation for decades.
"Unfortunately to confirm that we're seeing a planet we would likely have to
wait decades to see another transit," said co-author Nia Imara of the
University of California at Santa Cruz. "And because of the uncertainties
about how long it takes to orbit, we wouldn't know exactly when to look."
Can the dimming have been caused by a cloud of gas and dust passing in front
of the X-ray source? The researchers consider this to be an unlikely
explanation, as the characteristics of the event observed in M51-ULS-1 are
not consistent with the passage of such a cloud. The model of a planet
candidate is, however, consistent with the data.
"We know we are making an exciting and bold claim so we expect that other
astronomers will look at it very carefully," said co-author Julia Berndtsson
of Princeton University in New Jersey. "We think we have a strong argument,
and this process is how science works."
If a planet exists in this system, it likely had a tumultuous history and
violent past. An exoplanet in the system would have had to survive a
supernova explosion that created the neutron star or black hole. The future
may also be dangerous. At some point the companion star could also explode
as a supernova and blast the planet once again with extremely high levels of
radiation.
Di Stefano and her colleagues looked for X-ray transits in three galaxies
beyond the Milky Way galaxy, using both Chandra and the European Space
Agency's XMM-Newton. Their search covered 55 systems in M51, 64 systems in
Messier 101 (the "Pinwheel" galaxy), and 119 systems in Messier 104 (the
"Sombrero" galaxy), resulting in the single exoplanet candidate described
here.
The authors will search the archives of both Chandra and XMM-Newton for more
exoplanet candidates in other galaxies. Substantial Chandra datasets are
available for at least 20 galaxies, including some like M31 and M33 that are
much closer than M51, allowing shorter transits to be detectable. Another
interesting line of research is to search for X-ray transits in Milky Way
X-ray sources to discover new nearby planets in unusual environments.
Reference:
Rosanne Di Stefano, A possible planet candidate in an external galaxy
detected through X-ray transit, Nature Astronomy (2021).
DOI: 10.1038/s41550-021-01495-w.
Tags:
Space & Astrophysics