When Rosanne Di Stefano looks at the night sky, she sees something no human has ever confirmed before: a world that might exist in another galaxy entirely. The astronomer and her team at the Center for Astrophysics | Harvard & Smithsonian have detected what may be the first exoplanet ever found outside the Milky Way, a Saturn-sized candidate floating 28 million light-years away in the Whirlpool Galaxy.
The discovery, published in Nature Astronomy, did not come from peering through visible light. Instead, Di Stefano and her colleagues turned NASA's Chandra X-ray Observatory toward distant galaxies and watched for telltale dips in X-ray brightness — the signature of a planet passing in front of an X-ray bright binary system. In one such system, called M51-ULS-1, the X-rays vanished entirely for three hours. "We are trying to open up a whole new arena for finding other worlds," Di Stefano said.
The technique is elegantly simple: when a planet transits a star, it blocks some light. But the X-ray emitting region near a black hole or neutron star is so small that a planet can block nearly all of it, making the signal unmistakable. Current optical methods, by contrast, must detect tiny flickers as a planet casts a minimal shadow across its star — a far harder ask at interstellar distances.
All previously known exoplanets and candidates have been found within our own galaxy, almost all less than 3,000 light-years from Earth. If this candidate is real, it would sit thousands of times farther away than any confirmed world beyond our solar system. The team searched 238 systems across three galaxies — 55 in M51, 64 in the Pinwheel Galaxy, and 119 in the Sombrero Galaxy — and found just this one promising candidate.
The candidate planet appears to orbit its binary partner at roughly twice Saturn's distance from our Sun, circling a compact object that formed when a massive star collapsed in a supernova. If the planet could survive that violence, it may face more: someday, its companion star could also explode, bathing the world in yet another blast of radiation.
Confirmation will not come soon. The planet's wide orbit means astronomers would need to wait roughly 70 years for another transit to observe. "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. "We think we have a strong argument, and this process is how science works."
That patience is part of what makes the discovery meaningful, said co-author Nia Imara of the University of California, Santa Cruz. The very delay in verification speaks to the enormity of what the team has glimpsed — a possible world so distant that humanity may need most of a century to shake its hand.