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Chandra Sees Evidence for Possible Planet in Another Galaxy
Whirlpool Galaxy

  • Astronomers have announced evidence for a possible planet outside of the Milky Way galaxy.

  • This "exoplanet" would be much farther away than any of the thousands of others scientists have found in our Galaxy in recent years.

  • This planet candidate was identified with NASA's Chandra X-ray Observatory that detected a temporary dimming in X-rays in a binary system.

  • Researchers interpret this dimming as a planet passing in front of an X-ray source around a neutron star or black hole orbiting a companion star.

Astronomers have found evidence for a possible planet candidate in the M51 ("Whirlpool") galaxy, potentially representing what would be the first planet seen to transit a star outside of the Milky Way. As reported in our latest press release, researchers used NASA's Chandra X-ray Observatory to detect the dimming of X-rays from an "X-ray binary", a system where a Sun-like star is in orbit around a neutron star or black hole. The authors interpret this dimming as being a planet passing in front of the neutron star or black hole.

The left panel of this graphic shows M51 in X-rays from Chandra (purple and blue) and optical light from NASA's Hubble Space Telescope (red, green, and blue). A box marks the location of the possible planet candidate, an X-ray binary known as M51-ULS-1. An artist's illustration in the right panel depicts the X-ray binary and possible planet. Material from the companion star (white and blue in illustration) is pulled onto the neutron star or black hole, forming a disk around the dense object (illustrated as red and orange). The material near the dense object becomes superheated, causing it to glow in X-ray light (white). The planet is shown beginning to pass in front of this source of X-rays.

Looking for the dimming of a star's light as something passes in front of it is called the transit technique. For years, scientists have discovered exoplanets using transits with optical light telescopes, which detect the range of light humans can see with their eyes and more. This includes both ground-based telescopes and space-based ones like NASA's Kepler mission. These optical light transit detections require very high levels of sensitivity because the planet is much smaller than the star it passes in front of, and, therefore, only a tiny fraction of the light is blocked.

Transit Animation

Animation Credit: NASA/CXC/A.Jubett

The scenario of a transit in an X-ray binary is different. Because a potential planet is close in size to the X-ray source around the neutron star or black hole, a transiting planet passing along Earth's line of sight could temporarily block most or all of the X-rays. This makes it possible to spot transits at greater distances — including beyond the Milky Way — than current optical light studies using transits. A separate graphic shows how X-rays from M51-ULS-1 temporarily decrease to zero during the Chandra observations.

While this is a tantalizing study, the case of an exoplanet in M51 is not ironclad. One challenge is that the planet candidate's large orbit in M51-ULS-1 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. There is also the possibility that the dimming of X-rays is due to a passing cloud of gas near the M51-ULS-1, though the researchers think the data strongly favor the planet explanation.

Graphic showing possible orbits
Illustration Credit: NASA/CXC/M. Weiss

The paper describing these results appears in the latest issue of Nature Astronomy and is available online. The authors are Rosanne DiStefano (CfA), Julia Berndtsson (Princeton), Ryan Urquhart (Michigan State University), Roberto Soria (University of the Chinese Science Academy), Vinay Kashap (CfA), Theron Carmichael (CfA), and Nia Imara (now at the University of California at Santa Cruz). 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.

Please join our researchers today, Monday October 25, 2021 at 2pm EDT for a live Twitter Q&A on these results at:

3D printable files of M51/Whirlpool Galaxy
Credit: Tactile Universe, edited by NASA/CXC/SAO

Click for large jpg
3D Print Files
(zipped .obj & .stl files)

These 3D printable files of M51/Whirlpool galaxy were created by the Tactile Universe and edited by the Chandra team to add the location of the exoplanet candidate. Select the 3D printer of your choice to make your own 3D print of the M51 galaxy.


Fast Facts for M51/Whirlpool Galaxy:
Credit  X-ray: NASA/CXC/SAO/R. DiStefano, et al.; Optical: NASA/ESA/STScI/Gendler; Illustration: NASA/CXC/M.Weiss
Release Date  October 25, 2021
Scale  Image is about 6 arcmin (49,000 light years) across.
Category  Normal Galaxies & Starburst Galaxies
Constellation  Canes Venatici
Observation Date  11 pointings between March 2000 and October 2012
Observation Time  232 hours 10 minutes (9 days 16 hours 10 min)
Obs. ID  353, 354, 1622, 3932, 13812-13816, 15496, 15553
Instrument  ACIS
Also Known As NGC 5194, NGC 5195
References DiStefano, R., et al., 2021, Nature Astronomy (Published); PDF Document
Color Code  X-ray: purple and blue; Optical: red, green, and blue
Distance Estimate  About 28 million light years
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