Macquarie University: Doomed planet's death spiral could reveal stellar secrets

A massive planet five times heavier than Jupiter is locked in an astronomical death spiral, completing an orbit around its star in just 16 hours – and Macquarie University researchers have calculated exactly how this astronomical catastrophe will unfold. "A 'hot Jupiter' is a class of high-mass gas-giant planets orbiting so close to their star that they become super-heated; they are rare and make up around two planets in a thousand," says Dr Jaime A. Alvarado-Montes, a Macquarie Research Fellow who led the international study published on 15 July in The Astrophysical Journal.

One year on this giant, gaseous planet is an extraordinarily short 16 hours; and with a mass nearly five times that of Jupiter, and almost twice Jupiter's size, TOI-2109b orbits its star at a distance even closer than Mercury's orbit around our Sun. "Just to put it into context – Mercury's mass is almost 6000 times smaller than Jupiter, but it still takes 88 days to orbit our Sun. For a huge gas giant such as TOI-2109b to fully orbit in 16 hours – it tells us that this is a planet located super-close to its star."

This exoplanet's extreme proximity to its host star makes it an ideal laboratory for studying 'orbital decay' — an event predicted by theory but rarely detected in exoplanet systems. The Macquarie-led research team breakthrough came from combining archival data from multiple ground-based telescopes with observations from two space-based telescopes:

NASA's TESS mission and the European Space Agency's CHEOPS satellite. By analysing transit timing data spanning from 2010 to 2024, the team found that subtle changes in the planet's orbit can be detected. "Using all of the data available for this planet, we were able to predict a small change in its orbit," Dr Alvarado-Montes says. "Then we verified it with our theory and with our planet evolution models, and our predictions matched the observations.

That's quite exciting." The match between theoretical predictions and observational data represents a significant achievement in exoplanet science.