First known 'disappearing planet' has a neighbor that makes it wobble like a top

UCLA-led astronomers have discovered the first known "disappearing planet" about 1,000 light-years away. The planet, TOI-2535 b, is a "hot Jupiter," gas giants that orbit extremely close to their host stars. TOI-2535 b whips around its star in only 8.25 days - 10 times faster than the 88 days that Mercury takes to go around our Sun once. Researchers observed TOI-2535 b transit its star multiple times over seven years, until they stopped seeing it altogether. But as they show in an article published in Astrophysical Journal Letters, the planet hasn't truly vanished. Instead, gravity from a neighboring planet is tilting its orbit.

TOI-2535 b was identified in images taken by NASA's Transiting Exoplanet Survey Satellite (TESS) space telescope of the TOI-2535 star system between April 2019 and March 2026. TESS discovers planets by measuring the small changes in a star's brightness that occur when a planet passes in front of its star, blocking a bit of its light. These events, known as transits, require a precise alignment between the star, planet and our line of sight to be noticeable.

TESS observed this region of the sky numerous times, allowing the astronomers to determine that the planet's transits did not follow a regular schedule like most other exoplanets but ran dramatically early or late by up to six hours. They also noticed that the transits were getting shorter and shallower over the years, until they failed to appear completely in 2026.

These clues indicate that the planet is being pulled by an unseen neighbor, whose orbit is tilted relative to that of TOI-2535 b. The astronomers' calculations show that the two planets make each other's orbits wobble like a top, causing each to circle their sun obliquely and vanish from view as the wobble tilts them out of alignment.

An 'extreme example'

"We think of the universe as resembling clockwork, where everything is regular, but that's not true. If there are multiple planets in a system, they can affect each other's orbits. We know of a few systems that are like this," said Samuel Yee, a 51 Pegasi b Postdoctoral Fellow at UCLA and the paper's corresponding author. "This is the most extreme example we've seen because the two planets are interacting so strongly. The planet's orbit is tilting so quickly that its transit disappeared while we were watching."

TOI-2535 b will next transit its star again sometime in 30 to 50 years, while its hidden neighbor may come into view sooner. The phenomenon is the same as the one that makes eclipses on Earth so rare and exciting.

Samuel Lee

"If the moon's orbit around the Earth was perfectly aligned with the Earth's orbit around the sun, we would see an eclipse every month," said UCLA astrophysicist and paper co-author Erik Petigura. "But because the moon's orbit is tilted, eclipses are less common. The tilt of the moon's orbit is a relic of its violent formation history, which resulted from a collision with the early Earth."

The discovery reveals some surprising information about how the planet might have formed. The international team of researchers, led by Yee and Petigura at UCLA, determined that TOI-2535 b is an extremely hot, Jupiter-sized planet based on the size of its transit and precise observations from the WIYN 3.5-meter telescope at Kitt Peak National Observatory in Arizona. Hundreds of hot Jupiters have been discovered so far, orbiting their stars closely, but they don't usually have massive neighbors close enough to affect their orbit.

Two possibilities

The discovery of hot Jupiters so close to their stars puzzled scientists at first because existing models of planet formation suggested these planets need a lot of ice to form. Yee and Petigura are working on ways to understand how hot Jupiters could migrate toward the star from farther away. They think there are two possibilities. The first involves a violent pathway where the planet gets excited into an elliptical orbit, such as by a near-collision with another object, then interactions with the star pull the planet in.

In the second scenario, the planet gets pulled in along with other planets in the protoplanetary disc. But this scenario is expected to leave a "flat" system, without orbital tilts between planets. The wobble of TOI-2535 b suggests that components of the system may have interacted with each other in ways that over time tilted the orbits of the planets, similar to how the moon's orbit was likely tilted by a violent event.

"Because TOI-2535 b has a close neighbor, it probably migrated by slowly spiraling in instead of getting flung in," Yee said. "We may be seeing a formation channel that doesn't happen very often but is very interesting."