Indian astronomers have discovered a planet that is bigger than Jupiter and is orbiting too close to a star in its system. The exoplanet is dubbed TOI 1789b, orbiting an ageing star that is 1.5 times that of our Sun and located 725 light-years away.

The discovery was done by the exoplanet search and study group at the Physical Research Laboratory (PRL), Ahmedabad led by Professor Abhijit Chakraborty. Astronomers used the Advanced Radial-velocity Abu-sky Search (PARAS) optical fibre-fed spectrograph to detect the planet.

This is the second such exoplanet discovery to come out of the PRL by scientists using PARAS at 1.2 m Mt. Abu telescope; the first exoplanet K2-236b, a sub-Saturn size at 600 light-years away, was discovered in 2018. Scientists observed the planet between December 2020 and March 2021.

“It is one of the few nearby evolved stars with a close-in planet. The detection of such systems will contribute to our understanding of mechanisms responsible for inflation in hot Jupiters and also provide an opportunity to understand the evolution of planets around stars leaving the main sequence branch,” astronomers said in a paper published in the journal Monthly Notices of the Royal Astronomical Society.

The continued observations of the planet revealed that its mass is 70 per cent and size about 1.4 times that of Jupiter. Follow-up measurements using TCES spectrograph in Germany showed that it was a unique system in which the planet orbits the host star in just 3.2 days, placing it very-very close to the star at a distance of 0.05 AU (roughly one-tenth the distance between Sun and Mercury).

Due to this close proximity of the planet to its host star, it is extremely heated with a surface temperature reaching up to 2000 Kelvin giving it an inflated radius one of the lowest density planets known. “The detection of such system enhances our understanding of various mechanisms responsible for inflation in hot Jupiters and the formation and evolution of planetary systems around evolving and ageing stars,” it added.

The Indian Space and Research Organisation (Isro) in a statement said that there are less than 10 such close-in systems known among the zoo of exoplanets known so far.

The new discovery comes just weeks after Indian astronomers found a new method to understand the atmosphere of a planet beyond our solar system. Astronomers from the Indian Institute of Astrophysics designed a method to study the atmosphere of these planets by observing polarisation signatures or variations in the scattering intensity of light using ground-based radars and observatories.

Astronomers have been scouring through the sky looking for planets that could show signs of habitability. Researchers have talked about finding signs of life on some of these exoplanets in the coming decade. The most promising among them is the Hycean worlds, which are classified as exoplanets beyond the solar system which have densities between those of rocky super-Earths and more extended mini-Neptunes.

The habitable zone of these exoplanets revolves around Sun-like stars, which are more numerous and observable than Earth-like planets.