Stellar Oddities

ELBM J0555–57Ab is a red dwarf that is part of a binary system 630 light-years away from us in the constellation Painter. The main star is a Sun-like star with a slightly higher mass, while the smaller star has a mass about 85 times the mass of Jupiter, thus just above the hydrogen fusion limit. But its uniqueness is something else! In fact, it is the smallest “real” star discovered so far. Its diameter is only 85 percent that of Jupiter, and about the same as that of Saturn (without rings). It is also one of the densest known main sequence stars, with an average density of 188 g/cm3, or 34 times that of Earth. This tiny star will probably live something like 10–15 trillion years!

As for the largest planet, well, the matter is more complicated. In fact, increasing the mass of a planet above 13 Jovian masses is likely to trespass into the domain of brown dwarfs (so-called “missed stars”) where an increase in mass causes a contraction in radius, not to mention that estimating the radius of a planet is more difficult than estimating the radius of a bright star.