GJ 4256 b

Lore & Background

GJ 4256 b was discovered in 2024 using the transit method. The host star was observed in September of 2019 and 2022 by the Transiting Exoplanet Survey Satellite (TESS), and also with the CARMENES instrument at the Calar Alto Observatory. To rule out false-positive transit signals from a potential companion star, observations were made with Palomar/PHARO, which found no companion stars. After the planet's detection, a search for additional planets revealed a transit signal designated TOI-6255.02, which would have an orbit of 14.48 days and was reported by citizen scientist Alton Spencer on the ExoFOP website. However, due to a lack of additional prominent signals and substantial flux from a nearby background star, this signal was likely a false-positive.

The planet's physical characteristics have been modeled using two approaches. A two-layer interior model with an iron core surrounded by a silicate mantle yields an iron core mass fraction (CMF) of 0.45 ±0.32. The SUPEREARTH model, which distinguishes between true iron mass fraction (Fe-CMF of 0.38 ± 0.15) possibly due to iron in the silicate mantle and other elements in the core, suggests the simpler two-layered model is more accurate. The planet magnetically interacts with its star in a manner analogous to the interaction between Jupiter and its moon Io.

Tidal deformation is extreme: the planet orbits so close to its star that it has nearly passed the Roche limit. Tidal forces have deformed it into a triaxial ellipsoid (egg-like shape) with its long axis approximately 10% longer than the short axis, and possibly 15% longer than predicted by Love theory. The planet experiences tidal orbital decay and is predicted to pass the Roche limit and be ripped apart and engulfed by the star in around 400 million years. It is likely not currently disintegrating, unlike examples such as KOI-2700 b, KIC 12557548 b, K2-22b, and BD+05 4868Ab, all of which have longer orbital periods. Evidence for disintegration—such as asymmetries in transit data or debris material up to a kilometer in size—has not been observed.

Reader's Guide

GJ 4256 b holds significance as one of the most tidally distorted terrestrial planets known, with its egg-like shape resulting from extreme gravitational forces. Its ultra-short orbital period of 0.23 days and proximity to its red dwarf star make it a key object for studying tidal deformation and orbital decay. The planet is a favorable target for investigating magnetic interactions between the star and its planet, which may cause interior melting and accelerate orbital decay. It is also a top target for phase-curve observations by the James Webb Space Telescope (JWST), offering potential insights into the planet's atmosphere and surface conditions. The planet's predicted fate—being ripped apart and engulfed by its star in about 400 million years—provides a natural laboratory for understanding the end stages of close-in exoplanets. The lack of evidence for current disintegration, despite its extreme environment, distinguishes it from known disintegrating planets and raises questions about the timescales and processes involved. Its discovery and characterization contribute to the broader understanding of terrestrial planet formation and evolution in extreme orbital configurations.

Did You Know?

• GJ 4256 b has an orbital period of only 0.23 days, making it an ultra-short period planet.
• The planet's long axis is about 10% longer than its short axis due to tidal deformation.
• It is predicted to be ripped apart and engulfed by its star in around 400 million years.
• A citizen scientist named Alton Spencer reported the TOI-6255.02 signal on the ExoFOP website.