TOI-3686 planets orbit in a rare 2:1 resonance

TOI-3686 hosts a hot Jupiter and a smaller sub-Neptune that orbit in a 2:1 resonance: the outer planet takes almost exactly twice as long to orbit as the inner one. Both planets show large shifts in their transit timings, and TOI-3686 is the only known hot Jupiter system in a truly resonant configuration.

Star system: TOI-3686 | Distance from the Sun: 841 light years

TOI-3686 has two planets in a rare orbital arrangement. The inner planet, TOI-3686 b, is a sub-Neptune, a planet smaller than Neptune, and completes an orbit in 2.54 days. The outer planet, TOI-3686 c, is a hot Jupiter that takes 5.078 days to orbit its star. Their periods place them deep in a 2:1 mean-motion resonance: the outer planet takes about twice as long to complete an orbit as the inner one. TOI-3686 is currently the only known hot Jupiter system in a truly resonant configuration.

The planets were identified through changes in the times they pass in front of their star, events called transits. NASA's TESS space telescope first detected TOI-3686 c, and a later search of TESS observations found transits from TOI-3686 b. The team combined space- and ground-based brightness measurements with radial velocity data from the NEID and MAROON-X instruments. Radial velocity measurements track a star's motion toward and away from Earth, which can reveal the gravitational pull of orbiting planets. The researchers used photodynamical modeling, a method that models the planets' motions alongside the observed transits, to characterize the system.

The transit times vary substantially for both planets. TOI-3686 c has peak-to-peak timing variations of 40 minutes, among the largest observed for a hot Jupiter. TOI-3686 b varies by as much as 10 hours, about 16 percent of its orbital period. The system's age is estimated at no more than 1 billion years, and its young age may be related to the planets' resonant arrangement. Similar systems may have formed through relatively calm evolution and entered resonance as they migrated through a disk of gas and dust. They may also represent the high-mass end of processes found in compact systems of smaller planets.

Key findings
TOI-3686 b is a sub-Neptune with a 2.54-day orbit.
TOI-3686 c is a hot Jupiter with a 5.078-day orbit.
The planets are deep in a 2:1 mean-motion resonance, with the outer planet taking about twice as long to orbit as the inner planet.
Transit timing variations reach 10 hours for TOI-3686 b and 40 minutes for TOI-3686 c.
TOI-3686 is the only currently known hot Jupiter system in a truly resonant configuration.

Scientific study

TOI-3686: A Hot Jupiter and Sub-Neptune In A 2:1 Mean-Motion Resonance (external link)