If you were to imagine a star system, you’d probably think of a giant star being orbited
by smaller planets.
And generally speaking, this is the standard for star systems across the universe.
However, solar systems come in many shapes and sizes.
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.
When we talk of ‘big’, what we are actually referring to is the volume of an object, not
its mass.
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.
Below this threshold, the object is classified as a Brown Dwarf, because it didn’t become
massive enough for nuclear fusion to take place in its core.
Lower masses than Brown Dwarves are simply planets, asteroids, and dust.
So, 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.
Although, 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 10-29 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 29 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 would stretch far beyond our own.
We have discovered exoplanets around neutron stars, however, 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.
Instead, they shed their outer layers in a planetary nebula, leaving behind the core
of the star, or what is known as a white dwarf.
Nuclear fusion does not take place in a white dwarf, meaning their luminosity comes solely
from stored thermal radiation.
This makes them quite dim stars, and they only get dimmer over time.
Planets that would have survived the star’s evolution from main sequence, to red giant,
to white dwarf, could well be larger than the small remnant white dwarf, which is typically
the radius of Earth.
However, due to a white dwarf’s dim nature, tiny radius and lower mass, no such exoplanet
has been confirmed, although this doesn’t mean they aren’t out there.
When it comes to main sequence stars, planets larger than them are tricky to find too.
We believe that main sequence stars make up the majority of stars in the universe.
These can be very big objects, as internal pressure from the reactions in the core keeps
the radius large.
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.
And we also believe the dividing line between a planet and a brown dwarf is around 13 Jupiter
masses.
So, one of our best bets to find a tiny star with a large planet is looking around the
red dwarf VHS 1256-1257.
Its exoplanet is one of the few exoplanets to be discovered using direct imaging.
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.
Unfortunately though, this means that while the mass of the exoplanet has been determined,
we can’t know of its radius for sure.
However, this is one of the most likely candidates that we know of.
Also, because of the margins of error involved, it could well be that either or both objects
are brown dwarves, meaning we can’t say if this is truly a star and a planet system.
So, 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.
Which leads on to the final question I wanted to cover here.
What is the biggest exoplanet that we know of?
The most likely answer is GQ Lupi b.
It is another directly imaged exoplanet, which again means we don’t have a good grasp on
its physical characteristics.
From the margins of error involved, it could be a brown dwarf, but it’s probably the
largest exoplanet that we know of.
Scientists have estimated its radius to be 3 times the size of Jupiter, but again there
are margins of error involved.
Other contenders to this throne would be DH Tauri b, and ROXs 42Bb.
So, there we have it.
Some of the biggest exoplanets that we know of.
And it’s certainly feasible that somewhere out there, there is a planet bigger than its
host star.
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