This month, a team of scientists announced that it found the first possible planet beyond our Milky Way. The far-out discovery is literally that: If confirmed, it’s thousands of times farther away, in the Whirlpool Galaxy, than any planet we’ve observed so far.
M51-ULS-1, thought to be similar to Saturn, orbits a binary system in a galaxy about 28 million light-years away. Every other known planet is inside the Milky Way, which is just 100,000-200,000 light-years wide.
The M51 or “Whirlpool” galaxy, as photographed by the Hubble Space Telescope.
X-Ray method was key to finding the distant planet in the Whirlpool Galaxy
To find M51-ULS-1, the team used NASA’s Chandra X-ray Observatory, a telescope specially designed to detect X-rays in very hot regions around the universe. It specializes in detecting exploded stars, galaxy clusters, and matter orbiting black holes.
Because the earth’s atmosphere absorbs X-rays, the telescope must orbit above it. The Smithsonian’s Astrophysical Observatory in Cambridge, Massachusetts, hosts the base that operates the satellite.
The “optical bench” at the Chandra X-Ray Observatory.
While finding “exoplanets,” or planets inside our galaxy, has become more or less common, this discovery is a breakthrough. The team calls M51-ULS-1 an “extroplanet,” and finding it could be the key to detecting others like it. The resulting paper appeared in this month’s edition of Nature Astronomy.
Rosanne Di Stefano of the Center for Astrophysics, Harvard & Smithsonian in Cambridge, Massachusetts, who led the study, said that the X-ray technique used by the team is critical to ongoing extroplanetary discoveries.
Artist’s rendering of a planet, like M51-ULS-1, orbiting a binary system. Photo: NASA/CXC/A.JUBETT
The artist’s rendering above shows the system, possible planet, and basis for the X-ray technique. A binary system involves two solar bodies orbiting each other. In this case, the neutron star or black hole (bright white object at center) pulls material from the white and blue companion star into itself, forming the red/orange disk. The planet, which appears dark, orbits them both.
The paper explains how the material near the dense object gets superheated, making it glow under white X-ray light. Then, when anything passes between the object and the telescope, the transmitted light dims — in other words, it causes an eclipse. The team observed the effect for about three hours. It estimates that the potential planet is about the size of Saturn.
“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.”
Confirmation? May take awhile
Making sure that the new extroplanet is indeed a planet may require some patience. Because of its apparently wide orbit, 70 years may pass before the candidate planet causes another eclipse.
“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.”
