TOI-1227 b

Lore & Background

TOI-1227 b was discovered around the pre-main-sequence star TOI-1227, which was first identified as a PMS star by the Gaia satellite. Without this prior identification, the exoplanet signal would have been disregarded as an eclipsing binary due to the V-shape of its transit. The host star is located north of the globular cluster NGC 4372, but much closer to Earth at about 101 parsecs; NGC 4372 is 5800 parsecs away. TOI-1227 is part of a subgroup of the Lower Centaurus Crux OB association, sometimes called the Musca group after the constellation Musca, where most of its members lie. The star has a visual magnitude of about 17 and shows lithium in its atmosphere, which should be depleted within 10–200 million years for M-dwarfs.

The planet is still hot from its formation, and this heat, combined with a hydrogen-dominated primary atmosphere, makes its atmosphere inflated. Evolutionary models suggest that TOI-1227 b will eventually evolve into a sub-Neptune within the next billion years. No other Jupiter-sized planet has been detected around mid- to late M-dwarfs, despite the deep transits such a planet would create.

Reader's Guide

TOI-1227 b is significant as one of the youngest transiting exoplanets known, providing a rare window into planetary evolution during the contraction phase. Its current inflated size—85% that of Jupiter—is a direct result of residual formation heat and a hydrogen-dominated atmosphere, and models predict it will shrink to a sub-Neptune within a billion years. This evolutionary trajectory offers a concrete example of how some gas giants may transform into smaller planets over time. The planet's detection around a mid- to late M-dwarf is also notable, as no other Jupiter-sized planet has been found around such stars despite their deep transits. Future research may include radial velocity follow-up in the near-infrared (optical is not possible) or measurement of spin-orbit alignment via the Rossiter–McLaughlin effect, which is less challenging. The host star's pre-main-sequence status and lithium abundance further anchor the system's youth, making TOI-1227 b a key object for understanding early planetary atmospheres and contraction.

Did You Know?

• The planet is currently 85% the size of Jupiter but will contract into a sub-Neptune within about a billion years.
• Without prior identification of the host star as a pre-main-sequence star, the transit signal would have been dismissed as an eclipsing binary.