Tour: Possible Evidence for First Planet in Another Galaxy
(Credit: NASA/CXC/A. Hobart)
[Runtime: 03:18]

With closed-captions (at YouTube)

Astronomers have announced that they have found evidence for the first possible planet discovered outside of our Milky Way galaxy. To do this, they used a technique looking for the temporary dimming of X-rays from an object with NASA's Chandra X-ray Observatory. This intriguing result could open up a new window for exploring exoplanets — that is, planets outside our Solar System — at greater distances than ever before.

Until now, astronomers have found all known exoplanets — defined as planets outside of our Solar System — and exoplanet candidates in the Milky Way galaxy. Most of these have been less than about 3,000 light-years from Earth.

This new Chandra study looks at a system in the Messier 51, or M51, galaxy. This galaxy is nicknamed the "Whirlpool" galaxy because of its spiral shape. It is located about 28 million light years from Earth.

One system in M51 in particular caught the attention of a research team. This system, known as M51-ULS-1, belongs to a class of objects that astronomers call X-ray binaries. In these systems, a massive star is in orbit around either a neutron star or a black hole. Because the neutron star or black hole has such a strong gravitational force, it pulls material from the companion star into a disk swirling around the denser object. These disks glow brightly in X-rays because the material becomes superheated by the forces applied to it.

Because the region producing bright X-rays is relatively small, a planet passing in front of it could block most or all of the X-rays. This is known as a transit. Astronomers have been using transits to discover exoplanets within the Milky Way using optical telescopes for many years. However, these optical light transit searches need to be extremely sensitive because the planet blocks only a tiny fraction of the light from the star. In principle, the X-ray transit technique could allow astronomers to find exoplanets at much greater distances because the X-rays can completely temporarily disappear.

While this is a tantalizing result, it is not proof of the existence of an exoplanet in M51. Due to the nature of the suspected orbit, astronomers estimate they would need to wait at least 70 years for another transit to occur and confirm this current finding.

Regardless, this result provides motivation for researchers to look for more of these planet candidates in other galaxies and in our own, using X-rays.

Quick Look: Chandra Sees Evidence for Possible Planet in Another Galaxy
(Credit: NASA/CXC/A. Hobart)
[Runtime: 00:45]

Have astronomers found the first planet outside the Milky Way galaxy?

NASA's Chandra X-ray Observatory has detected possible evidence of an extragalactic exoplanet.

The temporary dimming of X-rays may be a planet passing in front of a neutron star or black hole.

This system is in the "Whirlpool" galaxy located 28 million light years away.

Zoom in on M51-ULS-1
(Credit: NASA/CXC/A. Hobart)
[Runtime: 00:36]

This artist's animation depicts the possible detection of a planet candidate in a double system in the M51 ("Whirlpool") galaxy. The view begins outside of the galaxy and then travels into the star cluster containing the binary system called M51-ULS. This contains either a neutron star or a black hole in orbit with a star about 20 to 30 times the mass of the Sun (large blue star). The neutron star or black hole is pulling material from its companion star, creating a disk of material that glows in X-rays (red, orange, and yellow). Astronomers using NASA's Chandra X-ray Observatory detected a decrease in the X-rays from M51-ULS over about a 3-hour period. They interpret this dimming as being a "transit," when a planet passing in front of the disk along the line of sight to Earth and blocks some or all of the light. This is depicted in the animation as the planet moves across the view from left to right and the bright light from material close to the neutron star or black hole briefly dims. By searching for exoplanets — that is, planets outside our Solar System — using this X-ray technique, scientists hope to look for these worlds at much greater distances than have previously been possible.

M51-ULS-1 Transit Only
(Credit: NASA/CXC/A. Hobart)
[Runtime: 00:09]

This artist's animation depicts the possible detection of a planet candidate in a double system in the M51 ("Whirlpool") galaxy called M51-ULS. This system contains either a neutron star or a black hole in orbit with a star about 20 to 30 times the mass of the Sun (large blue star). The neutron star or black hole is pulling material from its companion star, creating a disk of material that glows in X-rays (red, orange, and yellow). Astronomers using NASA's Chandra X-ray Observatory detected a decrease in the X-rays from M51-ULS over about a 3-hour period. They interpret this dimming as being a "transit," when a planet passing in front of the disk along the line of sight to Earth and blocks some or all of the light. This is depicted in the animation as the planet moves across the view from left to right and the bright light from material close to the neutron star or black hole briefly dims. By searching for exoplanets — that is, planets outside our Solar System — using this X-ray technique, scientists hope to look for these worlds at much greater distances than have previously been possible.