The exoplanet with the longest year discovered so far detected in the TOI-4600 system

Artist's concept of the TOI-4600 system (Image courtesy Tedi Vick)
Artist’s concept of the TOI-4600 system (Image courtesy Tedi Vick)

An article published in the “Astrophysical Journal Letters” reports the discovery of two exoplanets orbiting the star TOI-4600 of which the outermost is the one with the longest year discovered so far. A team of researchers used observations conducted with NASA’s TESS space telescope to find traces of these two gas exoplanets. Other ground-based observatories were used to confirm the existence of these two exoplanets and obtain more information about them. TOI-4600 b has a year of nearly 83 Earth days, a bit shorter than Mercury’s, while TOI-4600 c has a year of nearly 483 days, the longest of all known exoplanets. This is a useful discovery to understand what other planetary systems are like because exoplanets close to their stars are much easier to discover while we still know very little about the ones far from their stars.

The techniques used to discover exoplanets work very well for the ones close to their stars but hardly allow to detect the ones that are far away. In a planetary system very similar to the solar system, a hypothetical Earth twin would be difficult to spot. The TESS space telescope and other exoplanet hunters spot them when they pass in front of their stars, so it takes a stroke of luck to detect the passage of an exoplanet with a much longer year than Earth’s. One such came in observations of the star TOI-4600.

Just over 700 light-years from Earth, the star TOI-4600 is a little smaller than the Sun and is still quite young, with an estimated age of about 2.3 billion years. This type of star could be ideal for hosting potentially habitable planets due to its stability and long life, but in this study, the two exoplanets discovered around it are gas giants, interesting for other reasons.

The exoplanet TOI-4600 b has an orbit comparable to Mercury’s with its year lasting nearly 83 Earth days. It has a radius that is almost seven times the Earth’s, somewhere between those of Neptune and Saturn. Its so-called equilibrium temperature is about 75° Celsius, high but much lower than the hot Jupiters and hot Neptunians that orbit near their stars. It may be interesting precisely because it’s not one of these hot gas exoplanets but neither is it among the cold ones like the solar system’s gas giants.

Exoplanet TOI-4600c has a year that lasts nearly 483 Earth days, making its discovery valuable to astronomers. It has a radius that is about nine and a half times the Earth’s, so it’s similar in size to Saturn. The TESS space telescope detected its traces on two occasions almost three years apart. Its equilibrium temperature is around -80° Celsius, so it’s also one of the coldest exoplanets discovered so far by TESS.

Most of the more than 5,000 exoplanets discovered so far have a year that lasts less than 40 Earth days, so the discovery of two of them with a much longer year in the same planetary system is very interesting. The TOI-4600 system could also host other exoplanets that have escaped detection for now.

Astronomers are looking for planetary systems similar to the solar system to better understand their formation processes, and each new discovery helps this kind of study. For this reason, the TOI-4600 system will certainly be the subject of more observations, also because a new transit of the exoplanet TOI-4600 c is predicted for October 16, 2023, and even citizen scientists who have fairly sophisticated instruments were invited to participate in the research.

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