Astronomers have watched a planet pull off an unprecedented vanishing act. For the first time, a star that regularly dimmed as a planet crossed its face has stopped showing those dips, yet researchers say the planet still exists and its return can be estimated.

From April 2019 until March this year, the TESS space telescope observed TOI-2535 dim whenever TOI-2535b passed between Earth and the star. The event repeated every 8.24 days until the once dependable signal suddenly stopped.

Astronomers do not believe TOI-2535b was destroyed. Instead, they say a neighboring planet changed its orbital plane enough to halt the visible transits, producing a phenomenon long expected but previously observed only in double star systems.

Most planetary systems resemble the Solar System because their planets largely orbit within the same plane. This arrangement naturally follows from their formation in protoplanetary disks, although the alignment is not always perfect.

Even the Solar System has slight orbital differences. That is why observers see Transits of Venus only about twice a century instead of every time Earth overtakes Venus along the inside track.

Each planet exerts a gravitational pull on the orbital planes of other planets. Those effects remain small in the Solar System because its planets are widely separated and largely aligned with Jupiter, the only planet massive enough to make a major difference.

Greater misalignment or closely spaced giant planets can produce much larger orbital changes. Because transits require a planet’s orbital plane to align almost perfectly with the line between observers and its star, only a modest shift can make the transits disappear.

Astronomers say this explains why TOI-2535 stopped dimming eight months ago. Its transits had gradually become shorter and shallower, suggesting TOI-2535b was crossing increasingly higher stellar latitudes from Earth’s perspective.

The transit schedule also revealed that another object was exerting a powerful influence. TOI-2535b crossed as much as six hours earlier or later than expected, providing evidence for the unseen planet TOI-2535c.

Small transit timing changes are common, but extreme variations are rare. A study examining 561 star systems identified only 15 in which the variations were large.

The changing orbit also indicates that the two planetary planes are angled by more than 5 degrees, according to Dr Samuel Yee of UCLA and other authors. “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,” Yee said in a statement.

“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.”

Yee and his colleagues used ground based telescopes to confirm the observations. They found that TOI-2535 is 14% less massive than the Sun and almost twice its age.

The unusually dramatic effect reflects both the angle between the planets and the mass of the unseen companion. Yee and his coauthors calculate that TOI-2535c must possess at least as much mass as Saturn and possibly much more.

TOI-2535b is a hot Jupiter, meaning it is comparable to Jupiter in size and probable mass but circles extremely close to its star. Hot Jupiters are relatively easy to detect and represent a substantial share of the more than 6,000 known exoplanets.

Such planets seldom have nearby companions, especially large ones. They are thought to form beyond their systems’ snowlines before migrating inward, a journey that they usually complete alone.

The two commonly proposed migration scenarios are difficult to reconcile with TOI-2535. One involves an encounter pushing a planet into a highly eccentric orbit before the star gradually draws it inward, while the other involves slowly decaying planetary orbits.

“Because TOI-2535b 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.”

The planet’s absence from view will not be permanent. Yee and his coauthors estimate that continuing gravitational adjustments will eventually bring TOI-2535b across its star again in 30 to 50 years, and the study is open access in Astrophysical Journal Letters.