The universe is vast — possibly even infinite — and within the scheme of things, our planet is tiny. Even in our own solar system, Earth is dwarfed by gas giants like Jupiter. But are there greater planets on the market? How much greater? What’s the largest planet we all know of?
The reply will depend on several aspects, including the way you define a planet. Even so, there are a couple of candidates for the biggest known planet. One among the biggest known as ROXs 42Bb, a gas giant orbiting a star about 460 light-years from Earth. It’s about nine times the mass of Jupiter and has a radius of about 2.5 that of Jupiter.
Thayne Currie, an associate professor of physics and astronomy on the University of Texas — San Antonio, told Space.com he thinks it’s unlikely that this planet is really the biggest known. Currie identified ROXs 42Bb from data from the Keck Space Telescope in 2013 (one other group independently identified the item around the identical time). There are known objects around the identical size as this exoplanet and even larger, Currie continued.
“There are a few planets which can be actually protoplanets, so that they’re still being assembled,” he said. “I might suspect those are literally larger.” These two protoplanets each orbit the star PDS 70 about 370 light-years from Earth and have a radius between two and 4 times that of Jupiter. One other candidate for the biggest planet, HAT-P-67 b, had a radius larger than two times that of Jupiter, which has similarities to ROXs 42Bb.
Why the uncertainty? One reason has to do with the various ways scientists measure the scale of exoplanets. ROXs 42Bb, as an illustration, was directly imaged — “seen” as an independent object using the Keck Telescope. The protoplanets orbiting PDS 70 were also directly imaged. Scientists haven’t any approach to directly measure the scale of those planets, so that they should infer their size based on other aspects like their brightness and patterns within the wavelengths of sunshine they offer off. Scientists use models to find out these items, and these models are usually not at all times 100% correct.
Other objects are detected using the transit method, which is when an object appears to cross in front of its host star during its orbit and temporarily dims the star. Exoplanets detected in this manner, like HAT-P-67 b, will be directly measured. So it is likely to be a greater bet that this planet has over twice the radius of Jupiter.
The opposite uncertainty comes from the problem of the way to define a planet. Though most individuals know that stars are very large and planets are much smaller, there is a middle ground — an object called a brown dwarf, which is just too small to be a star but is larger than a planet. Though the core of a brown dwarf is just not hot enough to fuse regular hydrogen like a star would, it may well fuse deuterium, a special type of hydrogen that incorporates a neutron.
Scientists agree that brown dwarfs are usually not planets. What’s less clear is the way to distinguish between the 2.
“Some people discover a strict cut off in mass,” said Currie. “So anything above 13 Jupiter masses is a brown dwarf and anything below is a planet.”
But newer observations have revealed that the universe doesn’t necessarily “agree” with this rule. Research by Currie and his colleagues highlights that the turnover between planet and brown dwarf can occur at a much higher mass — perhaps 25 times the mass of Jupiter or much more massive. It also seems that it’s more necessary how massive an object is as compared to its host or companion star, said Currie.
Even then, there are some complexities. For example, Currie says that although he would call ROXs 42Bb a planet (or a “planetary-mass companion”), he suspects its formation was more just like how stars form. Typically, planets like Jupiter form a rocky core, which attracts a disk of dust and gas that step by step becomes a globular planet. ROXs 42Bb could have formed otherwise, where parts of the dust and gas disk were so massive and heavy that they collapsed in on themselves.
The way in which an object forms is just not currently a component of the formal definition of a planet. Some scientists confer with planetary-mass companions that formed like this as “sub-brown dwarfs,” though Currie said he “wouldn’t call that a thing.” He said scientists disagree on what to call ROXs 42Bb due to its high mass ratio (its mass in comparison with the mass of its star) and the way far-off from that star — over five times the gap between our sun and Neptune.
Though the talk over what “counts” as a planet could appear arbitrary, it highlights big questions on what different planetary systems might appear like, particularly those vastly different from ours, said Currie.
“Our own solar system is just one among many innumerable outcomes,” he said. “So it’s sort of fun to see how a planetary system might be different.”