Astronomers Search for Planets in Another Galaxy
Using NASA's TESS spacecraft, astronomers have begun a survey to find exoplanets in the Sagittarius Dwarf Galaxy Stream, a remnant of a galaxy merging with the Milky Way. Researchers have identified candidate planet signals in these extragalactic star streams. If confirmed, they would represent the first robust detection of planets that originated outside our own galaxy.
- The Sagittarius Dwarf Spheroidal Galaxy was discovered in 1994 and is one of the Milky Way's closest satellite galaxies, currently located about 70,000 light-years from Earth. It is in a looping, polar orbit and is being gradually disrupted and absorbed by the Milky Way. - NASA's TESS (Transiting Exoplanet Survey Satellite) mission, which launched in 2018, primarily hunts for planets using the transit method, which detects the slight dimming of a star's light as a planet passes in front of it. As of early 2026, the mission had identified nearly 8,000 planet candidates. - Previous claims of extragalactic planets have been made, but none have been unambiguously confirmed. A notable candidate announced in 2010, HIP 13044 b, was later retracted after further analysis revealed errors in the original data. - A more recent candidate, M51-ULS-1b, was identified in 2021 in the Whirlpool Galaxy, approximately 28 million light-years away. This potential planet was detected by a different method: observing a temporary blockage of X-rays from a binary system. - Confirming planets at such extreme distances is a major challenge because their host stars are too faint for follow-up observations with the radial velocity method. That technique, which has confirmed thousands of exoplanets, measures the gravitational "wobble" a planet induces on its star. - The Sagittarius dwarf galaxy is primarily composed of older, metal-poor stars known as Population II stars. Studying whether planets can form around such stars provides crucial data on the diversity of planetary systems under different galactic conditions. - The TESS satellite's primary mission is to find planets around the nearest and brightest stars to enable follow-up characterization by telescopes like the James Webb Space Telescope. Its wide-field cameras survey a new strip of the sky every 27 days, allowing it to cover nearly the entire sky over its extended missions.
