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.
A composite image of M51
with X-rays from Chandra and optical light from NASA's Hubble Space Telescope
contains a box that marks the location of the possible planet candidate. Credits:
X-ray: NASA/CXC/SAO/R. DiStefano, et al.; Optical: NASA/ESA/STScI/Grendler
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.
The other authors of this Nature Astronomy paper are
Ryan Urquhart (Michigan State University), Roberto Soria (University of the
Chinese Science Academy), Vinay Kashap (CfA), and Theron Carmichael (CfA).
NASA's Marshall Space Flight Center manages the Chandra program. The
Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science
from Cambridge Massachusetts and flight operations from Burlington,
Massachusetts.
Image credit: X-ray: NASA/CXC/SAO/R. DiStefano,
et al.; Optical: NASA/ESA/STScI/Grendler; Illustration: NASA/CXC/M.Weiss
Read more from NASA's Chandra X-ray Observatory.
For more Chandra images, multimedia and related
materials, visit: http://www.nasa.gov/chandra
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