Astronomers have just found a cosmic cannonball.
Around a star some 730 light-years away orbits an exoplanet
the size of Jupiter, but with a density that boggles the mind. Astronomers have
determined that the world, named TOI-4603b, has a mass of nearly 13 Jupiters.
That means it's nearly 3 times the density of Earth, and
just over 9 times the density of Jupiter. And it's really cuddly with its star,
with a tight orbit of just 7.25 days.
This places it in a small but significant category of worlds
that defy our understanding of planetary formation and evolution. This
discovery, accepted for publication in Astronomy & Astrophysics Letters, is
available on preprint server arXiv.
"It is one of the most massive and densest transiting
giant planets known to date," write a team of astronomers led by Akanksha
Khandelwal of the Physical Research Laboratory in India, "and a valuable
addition to the population of less than five massive close-in giant planets in
the high-mass planet and low-mass brown dwarf overlapping region that is
further required for understanding the processes responsible for their
formation."
Theoretically, there's a limit to how much mass a planet can
have. That's because, above a certain critical limit, the temperature and
pressure placed on the core is sufficient to ignite nuclear fusion, the process
of smushing together atoms to create heavier elements.
For a star, the minimum mass at which this process starts is
around 85 Jupiters; at that point, hydrogen atoms start fusing into helium.
The upper mass limit for a planet is thought to be around 10
to 13 Jupiters. And the objects that straddle the gap between them are known as
brown dwarfs. These don't have enough mass for hydrogen fusion; however, their
cores can fuse deuterium, a heavy isotope of hydrogen that doesn't quite need
as much heat and pressure.
Stars form from the top down, when a dense clump in a
molecular cloud collapses under gravity to form a protostar. The star then
grows by slurping up material from the cloud around it, which arranges into a
disk.
The dust and gas left over after this process forms planets,
which start from the bottom up as bits of rubble start to stick together,
eventually forming clumps that grow into planets.
Brown dwarfs are thought to form like stars, from a clump of
molecular cloud that collapses under gravity. They're usually found orbiting
stars at quite a wide separation, at a minimum of a five astronomical units
(AU) – that's five times the distance between Earth and the Sun.
Astronomers believe that they form in a similar way to
stars, collapsing from a clump of material in a cloud, and there's a remarkable
"desert" of brown dwarfs with close orbits.
TOI-4603b was first spotted in data from NASA's
exoplanet-hunting space telescope TESS, which studies patches of the sky
looking for faint, regular dips in starlight that suggest the presence of an
orbiting exoplanet. The TESS data suggested a world 1.042 times the radius of
Jupiter, whipping around its star in just over a week.
The team followed up seeking radial velocity measurements.
That's the amount by which an exoplanet's gravity moves its host star, as the
two bodies orbit a mutual center of gravity. If you know the mass of the star,
you can work out the mass of the exoplanet by calculating how much the star is
moving around.
This is how the researchers derived a mass for TOI-4603b of
12.89 times the mass of Jupiter. Combining that with the object's radius
allowed the team to arrive at an average density of 14.1 grams per cubic
centimeter. Earth's density, for context, is 5.51 grams per cubic centimeter.
Jupiter's is 1.33 grams per cubic centimeter. Lead has a density of 11.3 grams
per cubic centimeter.
That's not at all strange, for a brown dwarf, which on
average is around 0.83 times the radius of Jupiter; one brown dwarf, with a
radius 0.87 times that of Jupiter, has a mass of around 61.6 Jupiters, for
example. These things can get much denser than TOI-4603b.
TOI-4603b fits most of the criteria to be classified as an
exoplanet, and that's what Khandelwal and her colleagues have called it. But
it's right on the cusp of the brown dwarf mass limit, which means that it could
be an important world for understanding how brown dwarfs and giant planets
form, and how their relationship to their stars evolve.
For example, the exoplanet has an orbit that is
significantly oval, or eccentric, suggesting that it is still settling into it.
And the star also has a brown dwarf companion, orbiting at around 1.8
astronomical units, which could have gravitationally interacted with TOI-4603b.
These clues suggest that the exoplanet is migrating closer to the star from a
more distant position.
A similar object is a world called HATS-70b, which at 12.9
times the mass of Jupiter and 1.384 times its radius, is less dense than
TOI-4603b, but similarly close to its star, and also shows signs of migration.
"Detection of such systems," the researchers
write, "will offer us to gain valuable insights into the governing
mechanisms of massive planets and improve our understanding of their dominant
formation and migration mechanisms."
The research has been accepted in Astronomy &
Astrophysics Letters, and is available on arXiv.