Astronomers Solve the Mystery of a Planet That Vanished

By Lisa F. Keith

A giant planet about 1,000 light-years away has not vanished after all, astronomers say. It has been knocked off course by an unseen neighbor, and that tug tilted its orbit so far that the planet no longer passes in front of its star as seen from Earth. The finding, published Monday in The Astrophysical Journal Letters, explains the first known case of a planet whose transits disappeared while scientists were watching. The planet, TOI-2535 b, is a hot Jupiter, a gas giant that circles extremely close to its star. It completes an orbit in about 8.25 Earth days, roughly ten times faster than Mercury circles the Sun. NASA’s Transiting Exoplanet Survey Satellite, known as TESS, tracked it from April 2019 to March 2026 by watching for the small dip in starlight each time the planet crossed the face of its star. For years the signal looked ordinary, then it began to misbehave. Transits arrived as much as six hours early or late, and they grew shorter and shallower. By March of this year, TESS stopped seeing them altogether. A team led by Samuel Yee and Erik Petigura at the University of California, Los Angeles, combined the satellite’s data with follow-up observations from the WIYN 3.5-meter telescope at Kitt Peak and concluded that the planet’s orbit was tilting out of line with our view. What Is Pulling on the Planet The best explanation is a second, massive planet whose orbit sits at an angle to the one TOI-2535 b follows. The two worlds tug on each other, so both orbits slowly wobble, much like a spinning top. Yee called it the most extreme case of this kind of interaction astronomers have seen, because the tilt changed so quickly that the transit disappeared within the observing window. The neighbor has not been detected directly, so its existence is an inference drawn from the timing shifts and the changing shape of the transits. The researchers estimate that TOI-2535 b will not cross its star again for at least 30 years. The hidden planet, however, could begin to transit sooner, which would give astronomers a chance to spot it directly. Why Hot Jupiters Puzzle Scientists Hot Jupiters have long been hard to explain. A gas giant needs a lot of cold, icy material to form, and there is very little of that so close to a star, so scientists think these planets are born farther out and then move inward. One route is violent: a near-collision with another body flings a planet into a stretched orbit, and the star’s gravity gradually pulls it closer. The other is gentle, with the planet drifting inward through the disk of gas and dust around a young star, which should leave the planets in a flat, well-aligned system. A tilted orbit would normally argue against that gentle path. The case of TOI-2535 b suggests a flat system can be twisted later in its life, which keeps the slow-migration idea alive. Because the planet has a close companion, Yee said, it probably spiraled in gradually instead of being thrown inward, and it may represent a formation route that rarely happens but is especially interesting. If the second planet is confirmed, the pairing would be a rare example of a hot Jupiter with a nearby giant companion. What Happens Next For now, astronomers will keep working out how the system formed and how to detect the hidden planet, since the repeating shadow they relied on is gone. Future study will combine several detection methods rather than lean on a single dip in starlight. The episode also shows that long-running surveys like TESS can catch surprises, including planets that change their behavior in real time. The 30-year estimate is a projection based on current orbital modeling, not a guarantee. Even so, the discovery gives astronomers a rare natural laboratory for testing how giant planets end up so close to their stars.