|Can a planet be bigger than a star.|
Are there any planets bigger than the sun?Could it be that there are planets out there that are bigger than the star they are orbiting? And leading on from that, how big can planets get? What's the biggest one that we know of? First of all, let's define one point.
In order for a star to be a star, it has to be over a certain mass, namely 0.08 solar masses, or 8% of our Sun's mass.
The minimum mass of a star is 0.08 M, is there also an upper limit? Theoretically, the most amount of mass a star can attain is about 150 M before models suggest it would lose its stability.
Interestingly enough, there are some stars in existence that seemingly contradict that theory, like the most massive star that we know of, R136a1, which is estimated to be around 300 M. Regardless, just because a star is massive, doesn't mean that it's big.
Some of the densest stars in existence, Neutron Stars, are 1-2 M and can be only 30km across.
Imagine fitting the vast mass of our star, an object so huge that it dwarfs Jupiter considerably, let alone us, and squeeze up to 2 of these into a space the size of a large city.
While these stars are tiny volume-wise, they easily have enough mass to host a grand solar system that could stretch far beyond our own.
We have discovered exoplanets around neutron stars due to the detection method used to discover them, we can only define their mass, not their radius.
It would make sense though that these planets are many times bigger than the tiny star, as we have asteroids in our Solar System which can be 100s of km across, let alone planets thousands of km across.
White dwarves are another example of tiny stars that likely have larger planets orbiting them.White dwarves are the remnants of red giant stars that weren't massive enough to become neutron stars in a supernova explosion when they reached the end of their lives.
This makes them quite dim stars, and they only get dimmer over time.
There has just been confirmation of such a planetary system thanks to the TESS observatory, an exoplanet known as WD 1856 b. What's remarkable about this system is that the planet orbits the white dwarf relatively closely, meaning it survived the star going through its red giant phase.
The red giant phase would have easily enveloped where the planet now resides, so this means the planet must have fallen or scattered into this orbit after the event, perhaps thanks to the gravitational influences of the star's other undiscovered planets, or even from nearby stars that the white dwarf is gravitationally bound to.
In any case, it's an incredible system indeed! When it comes to main sequence stars, planets larger than them are tricky to find too.
We'd have to look for stars right around the dividing line between brown dwarves and red dwarves to find a main sequence star small enough to host a larger planet.
Remember, the difference between a brown dwarf and a main sequence star is whether nuclear fusion takes place in the star's core, which we believe happens above 0.08 solar masses.
One of our best bets to find a tiny star with a large planet is looking around the red dwarf VHS 1256-1257.
Its host star is dim enough, the exoplanet far enough away from the star, and the exoplanet big enough to be seen simply by using a powerful telescope.
Because of the margins of error involved, it could well be that either or both objects are brown dwarfs, meaning we can't say if this is truly a star and a planet system.
While we don't have definitive proof of a planet being bigger than its host star, we have some promising candidates, and there's almost certainly cases out there that we just haven't found yet.
Some of the smallest stars out there have a radius of roughly 70,000km, some of the biggest planets out there can be double or triple that.
It's certainly feasible that somewhere out there, there is a planet bigger than its host star.