A Jupiter-sized exoplanet orbits a star about 730 light-years away, but with a mind-boggling density. Astronomers determined that the earth, designated TOI-4603b, has about 13 Jupiter masses. This means its density is almost 3 times that of Earth and just over 9 times that of Jupiter. And it’s really beautiful, with a star with a narrow orbit of just 7.25 days.

This places it in a small but important world category that challenges our understanding of planet formation and evolution. The discovery, accepted for publication in Astronomy & Astrophysics Letters, is available on the arXiv preprint server.

“This is one of the largest and most densely transiting giant planets known to date,” writes a team of astronomers led by Akanksha Khandelwal of the Physical Research Laboratory in India, “a valuable addition to the population of less than five nearby planets.” In giant planets in the region of overlap between a high-mass planet and a low-mass brown dwarf, this is also essential for understanding the processes responsible for their formation.”

Theoretically, there is a limit to the mass of a planet. This is because above a certain critical limit, the temperature and pressure applied to the core is sufficient to ignite nuclear fusion, which is the process of bringing atoms together to form heavier elements. The minimum mass at which this process begins for a star is about 85 Jupiter; At this point, the hydrogen atoms begin to turn into helium.

The upper limit of the planet’s mass is believed to be between 10 and 13 Jupiter masses. Objects that cross the space between them are known as brown dwarfs. They do not have enough mass to synthesize hydrogen; but their cores can fuse deuterium, a heavy isotope of hydrogen that doesn’t require much heat and pressure. Stars form from top to bottom when a dense cluster in a molecular cloud collapses under the influence of gravity to form a protostar. The star then grows by absorbing material that forms the disk from the surrounding cloud.

The dust and gas left over from this process form planets as debris begins to stick together, forming clumps that start from the bottom up and eventually turn into planets. Brown dwarfs are thought to form from a cluster of molecular clouds that collapse under the influence of gravity, like stars. It is usually at a fairly large distance orbiting the stars, at least five astronomical units (au) – five times the distance between the Earth and the Sun.

Astronomers believe they formed by collapsing from a clump of material in a cloud, like stars, and there is an incredible “desert” of brown dwarfs with close orbits.

TOI-4603b was first detected in data from NASA’s TESS space telescope, which scanned the sky for weak, regular dips in starlight that would indicate the presence of an orbiting exoplanet. TESS data showed that Earth, 1,042 times the radius of Jupiter, orbited its star in just over a week.

The team tried to measure the radial velocity. This is the amount by which the exoplanet’s gravity moves the host star when the two objects orbit a common center of gravity. If you know the mass of the star, you can determine the mass of the exoplanet by calculating how much the star has moved.

This is how the researchers determined the mass of TOI-4603b, which is 12.89 times the mass of Jupiter. Combining this with the radius of the object allowed the group to achieve an average density of 14.1 grams per cubic centimeter. In terms of context, Earth’s density is 5.51 grams per cubic centimeter. Jupiter is 1.33 grams per cubic centimeter. The density of lead is 11.3 grams per cubic centimeter.

For a brown dwarf about 0.83 times the radius of Jupiter, this is not surprising; For example, a brown dwarf with a radius of 0.87 times that of Jupiter has a mass of about 61.6 Jupiter. These things can be much denser than TOI-4603b.

TOI-4603b meets most of the criteria for classification as an exoplanet, the name Khandelwal and colleagues gave it. But it’s on the mass limit of a full brown dwarf, which means it could be an important world for understanding how brown dwarfs and giant planets form and how their relationships with stars evolve.

For example, an exoplanet’s orbit is significantly oval or eccentric, indicating that it is still sitting on it. The star also has a brown dwarf companion orbiting about 1.8 AU, which may have gravitationally interacted with TOI-4603b. These clues suggest that the exoplanet is migrating closer to the star from a more distant position.

A similar object is a world called HATS-70b, which has a mass of 12.9 times Jupiter and a radius of 1,384 times, less dense than TOI-4603b but just as close to its star, and also showing signs of migration.

“The discovery of such systems will provide us with valuable insights into the governing mechanisms of giant planets and will improve our understanding of their dominant formation and migration mechanisms,” the researchers write.