The scientific community still does not know exactly which is the largest planet ever recorded due to the difficulty in measuring the mass and size of exoplanets.
Jupiter is the largest planet in the Solar System and has been known as the largest planet for nearly 400 years. However, with the countless exoplanets discovered, Jupiter’s size record has been broken multiple times, according to IFL Science.
The majority of the controversy when the International Astronomical Union (IAU) created a definition for planets in 2005 focused on celestial bodies that were excluded, particularly Pluto. We define a planet as a celestial body that orbits a star but is not massive enough to initiate nuclear fusion in its core. Other conditions include it must achieve hydrostatic equilibrium (the planet must be large enough for its gravity to shape it into a spherical or near-spherical form).
Simulation of the planet GQ Lupib (right) compared to Jupiter (left). (Photo: Eurotek).
The issue with large celestial bodies is that they are more likely to fuse deuterium than hydrogen. This creates a category of celestial bodies known as brown dwarfs, which have enough pressure to fuse deuterium atoms in their cores but not hydrogen. Since deuterium is very rare, brown dwarfs emit light that is a tiny fraction of a complete star, making them hard to distinguish from ordinary planets. Some astronomers classify brown dwarfs and white dwarfs as planets. However, the IAU’s definition excludes both.
Even after narrowing down the celestial bodies that can be classified as planets, researchers still do not know exactly which is the largest planet because estimates of the mass and size of exoplanets are somewhat uncertain. They are not even confident in classifying many celestial bodies as planets or brown dwarfs.
GQ Lupib is a potential candidate for the title of the largest planet, depending on whether it can fuse deuterium. Researchers have attempted to estimate its radius at least four times, yielding various values from 1.8 to 4.6 times the radius of Jupiter. This gigantic planet could be up to 200 times the volume of Jupiter. The reason GQ Lupib is difficult to estimate in size is that the planet is very young. At that age, planets still retain a lot of heat from previous gravitational potential. GQ Lupi b has a temperature of about 2,000 degrees Celsius, making it difficult for scientists to determine whether all the heat comes from the planet’s formation process or from ongoing nuclear fusion. Furthermore, the planet is too far from its host star (3.3 times the distance from Neptune to the Sun) for us to measure its mass through observations of its gravitational influence.
Another candidate is PDS 70b, with estimates indicating its radius is 2.72 times that of Jupiter. PDS 70b could become the planet with the largest volume if the possibility of it being a brown dwarf is ruled out.
In contrast to the aforementioned exoplanets, HAT-P-67b transits in front of its host star and can be observed from Earth. It has an estimated radius of 2.085 times that of Jupiter with a very small margin of error. However, HAT-P-67b is certainly not the largest planet. Its mass is closer to that of Saturn than to Jupiter, revealing a very low density due to its proximity to the host star. Only a few planets have estimated sizes similar to HAT-P-67b, but with large margins of error.
