Astronomers have made a surprising discovery: planets that are twice as wide and more than ten times the mass of Earth, yet they still have certain limits in terms of their size.
The exoplanet Kappa Andromedae b has a mass 12.8 times that of Earth. Image: NASA.
Ever since astronomers first observed celestial bodies beyond the Solar System three decades ago, in their quest to detect exoplanets, humans have known that planets within the Milky Way galaxy, and the universe at large, come in a wide range of diameters and masses. Prior to 1992, when the first exoplanet was discovered, Jupiter, a massive gas giant with a diameter 11 times that of Earth, held the record as the largest known planet. However, Jupiter’s size appears rather modest compared to some of the colossal planets discovered by humans to date, according to Live Science.
When determining the size of a planet, two key measurements are considered: diameter and mass. According to the first measurement, the largest exoplanet has a radius twice that of Jupiter, as stated by Solène Ulmer-Moll, a postdoctoral researcher at the University of Geneva. This celestial body orbits very close to its host star. The width and mass of a planet are related, but they do not always have a direct correlation. This is because planets exhibit a wide range of densities. Some massive gas giants can be larger than solid, dense planets.
For instance, the gas giant exoplanet HAT-P-67 b boasts a radius twice that of Jupiter, currently ranking among the widest planets. However, despite its size, this exoplanet located 1,200 light-years from Earth has a very low density, making it only one-third as massive as Jupiter, according to Ulmer-Moll. Another example is WASP-17 b, which is also twice the size of Jupiter. The next in line is KELT-9b, with a radius 1.84 times that of Jupiter.
The majority of rocky planets never reach the colossal proportions of the “super-Jupiters” mentioned above. The largest rocky planets, often referred to as “super-Earths,” have a radius twice that of Earth. For instance, Wasp-17b has a radius 22 times that of Earth. While rocky planets are denser than gas giants, they still cannot match them in terms of mass. This is because, during their development, rocky planets accumulate gases and ice.
The largest planets have a mass approximately 13 times that of Jupiter. For example, the gas giant HD 39091 b, which is located 60 light-years from Earth, is 12.3 times more massive than Jupiter. Researchers believe that it is unlikely to discover planets larger than these, as a planet would become a “brown dwarf” upon reaching a certain size and mass threshold.
Brown dwarfs are often likened to “failed stars” because they are heavier than super-Jupiters but not large enough to trigger nuclear fusion reactions in their cores. “The key difference between a brown dwarf and a planet is mass and the deuterium-burning process, the heavy isotope of hydrogen,” explained Nolan Grieves, a postdoctoral researcher at the University of Geneva. “At higher masses, an object will have enough pressure and temperature inside to burn off most of the deuterium it has.”
The largest known brown dwarf is SDSS J0104+1535, which is located 750 light-years from Earth at the edge of the Milky Way. It has a mass 90 times that of Jupiter but a radius that ranges from 0.7 to 1.4 times that of Jupiter. In reality, it is still smaller than the largest planet in the Solar System. (Source: Live Science)