TOI-2298 b orbits in its star's habitable zone

TOI-2298 b is a planet about three times Earth’s radius, orbiting its star every 146.58 days in the conservative habitable zone. Its orbit was clarified by checking years of TESS observations for repeat transits. A wider review of 187 objects also found that many planets labelled as habitable-zone candidates from incomplete transit records are probably closer to their stars.

Star system: TOI-2298 | Distance from the Sun: 275 light years

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TOI-2298 b orbits in its star's habitable zone

Star system: TOI-2298 | Distance from the Sun: 275 light years

A revised orbit places TOI-2298 b in the conservative habitable zone around its star. The planet is a sub-Neptune, meaning it is larger than Earth but smaller than Neptune, with a radius about 3.0 times Earth’s. It takes 146.58 days to complete an orbit. The conservative habitable zone is the range where a planet could receive an amount of starlight compatible with liquid water on its surface, though that alone does not establish that a planet is habitable.

The orbit was determined by examining public data from NASA’s Transiting Exoplanet Survey Satellite, or TESS, through Sector 107. A transit occurs when a planet passes in front of its star and briefly dims its light. When only one or two transits are observed, the time between them may not reveal the planet’s true orbital period. The analysis compared possible periods, weighing how well each matched the observed dips in starlight and checking for misleading signals from data gaps, other planets, or instrument effects.

The broader review examined 187 TESS Objects of Interest with undefined periods or catalogue periods of at least 200 days. It resolved 28 periods and ruled out possibilities for eight more. The results show that many objects labelled as being in the habitable zone based on catalogue periods are likely on shorter, hotter orbits. Of 28 objects whose two observed transits appeared to indicate a conservative habitable-zone orbit, only about two are expected to truly lie there, and both are giant planets. None of the 12 planet-sized objects in that group are expected to be there.

At the same time, incomplete or incorrect periods can hide planets that do orbit in the habitable zone. The analysis recovered three previously reported examples around red dwarfs and validated four planets using newly determined periods. It also found that TOI-6075 b has a radius about 1.7 times Earth’s and an orbital period of 416.5 days; an 833-day period could not be ruled out. It is the second validated transiting planet smaller than twice Earth’s radius with an orbital period longer than one year. The results include revised probabilities of habitable-zone membership for all 187 objects.

Key findings
TOI-2298 b has a radius about 3.0 times Earth’s and orbits every 146.58 days in the conservative habitable zone.
Of 28 two-transit objects that appeared to be in the conservative habitable zone, about two are expected to truly be there; both are giant planets.
The analysis resolved 28 orbital periods and ruled out additional possibilities for eight objects in a sample of 187.
TOI-6075 b is about 1.7 times Earth’s radius and has a 416.5-day period, although an 833-day period remains possible.

Scientific study

Are habitable-zone TESS Objects of Interest really there? Period resolution from the full archive (external link)