Lund University astronomers discover a space oddity

A research team led by Lund University in Sweden has discovered a small planet that displays peculiar orbital motion. The shimmying planet, located 455 light-years from Earth, shows that planetary systems can be considerably more complex than researchers have previously thought.

Photo credit: ESO/M. Kornmesser

Lund University astronomers discover a space oddity - an exoplanet moving in mysterious ways

The newly discovered planet TOI-1408c has a mass equivalent to eight Earths and circles very close to a larger planet, the hot gas giant TOI-1408b. After starting to study both planets and their star, TOI-1408, in detail, the researchers felt puzzled. The small planet has a very peculiar orbital motion. The interactions between the two planets and their star can be likened to a rhythmic dance.

"The small planet exhibits very unusual orbital behaviour and shows considerable variations regarding the time when it passes in front of its star, which is something that we don’t see as a rule. The small planet’s existence challenges existing theories on the formation and stability of planetary systems," says Judith Korth, astrophysicist at Lund University and leader of the study.

TOI-1408: Discovery and Photodynamical Modeling of a Small Inner Companion to a Hot Jupiter Revealed by Transit Timing Variations

The new study, published in The Astrophysical Journal Letters, shows that planetary systems can be considerably more complex than researchers have previously thought. The discovery of a small planet between a star and a gas giant is rare and offers a unique case study for the development of planetary systems. This could help the researchers to understand more about how planets are formed in other solar systems.

"Our results will help researchers to learn more about how planets are formed and how they behave when they are very close to each other, particularly in systems with giant planets," says Judith Korth.

Exoplanets are planets located in a solar system other than our own. The first confirmed discovery was made in 1995. Since then, over 5,700 exoplanets have been discovered. The researchers’ discovery of the space oddity TOI-1408c was made possible by using NASA’s Transiting Exoplanet Survey Satellite (TESS). Since TESS was launched in 2018, it has observed over 7,000 potential exoplanets.

"I hope that our results can be used in future studies to discover even more planets in other systems, but also to better understand the large range of planetary systems that exist in our galaxy," concludes Judith Korth.

Abstract

We report the discovery and characterization of a small planet, TOI-1408 c, on a 2.2 day orbit located interior to a previously known hot Jupiter, TOI-1408 b (P = 4.42 days, M = 1.86 ± 0.02 M_Jup, R = 2.4 ± 0.5 R_Jup) that exhibits grazing transits. The two planets are near 2:1 period commensurability, resulting in significant transit timing variations (TTVs) for both planets and transit duration variations for the inner planet. The TTV amplitude for TOI-1408 c is 15% of the planet's orbital period, marking the largest TTV amplitude relative to the orbital period measured to date. Photodynamical modeling of ground-based radial velocity (RV) observations and transit light curves obtained with the Transiting Exoplanet Survey Satellite and ground-based facilities leads to an inner planet radius of 2.22 ± 0.06 R_⊕ and mass of 7.6 ± 0.2 M_⊕ that locates the planet into the sub-Neptune regime. The proximity to the 2:1 period commensurability leads to the libration of the resonant argument of the inner planet. The RV measurements support the existence of a third body with an orbital period of several thousand days. This discovery places the system among the rare systems featuring a hot Jupiter accompanied by an inner low-mass planet.

Citation Judith Korth et al 2024 ApJL 971 L28