Astronomers have developed the most realistic model to date of planet
formation in binary star systems.
The researchers, from the University of Cambridge and the Max Planck
Institute for Extra-terrestrial Physics, have shown how exoplanets in binary
star systems—such as the 'Tatooine' planets spotted by NASA's Kepler Space
Telescope—came into being without being destroyed in their chaotic birth
environment.
They studied a type of binary system where the smaller companion star orbits
the larger parent star approximately once every 100 years—our nearest
neighbour, Alpha Centauri, is an example of such a system.
"A system like this would be the equivalent of a second Sun where Uranus is,
which would have made our own solar system look very different," said
co-author Dr. Roman Rafikov from Cambridge's Department of Applied
Mathematics and Theoretical Physics.
Rafikov and his co-author Dr. Kedron Silsbee from the Max Planck Institute
for Extra-terrestrial Physics found that for planets to form in these
systems, the planetesimals—planetary building blocks which orbit around a
young star—need to start off at least 10 kilometres in diameter, and the
disc of dust and ice and gas surrounding the star within which the planets
form needs to be relatively circular.
The research, which is published in Astronomy and Astrophysics, brings the
study of planet formation in binaries to a new level of realism and explains
how such planets, a number of which have been detected, could have formed.
Planet formation is believed to begin in a protoplanetary disc—made
primarily of hydrogen, helium, and tiny particles of ices and dust—orbiting
a young star. According to the current leading theory on how planets form,
known as core accretion, the dust particles stick to each other, eventually
forming larger and larger solid bodies. If the process stops early, the
result can be a rocky Earth-like planet. If the planet grows bigger than
Earth, then its gravity is sufficient to trap a large quantity of gas from
the disc, leading to the formation of a gas giant like Jupiter.
"This theory makes sense for planetary systems formed around a single star,
but planet formation in binary systems is more complicated, because the
companion star acts like a giant eggbeater, dynamically exciting the
protoplanetary disc," said Rafikov.
"In a system with a single star the particles in the disc are moving at low
velocities, so they easily stick together when they collide, allowing them
to grow," said Silsbee. "But because of the gravitational 'eggbeater' effect
of the companion star in a binary system, the solid particles there collide
with each other at much higher velocity. So, when they collide, they destroy
each other."
Many exoplanets have been spotted in binary systems, so the question is how
they got there. Some astronomers have even suggested that perhaps these
planets were floating in interstellar space and got sucked in by the gravity
of a binary, for instance.
Rafikov and Silsbee carried out a series of simulations to help solve this
mystery. They developed a detailed mathematical model of planetary growth in
a binary that uses realistic physical inputs and accounts for processes that
are often overlooked, such as the gravitational effect of the gas disc on
the motion of planetesimals within it.
"The disc is known to directly affect planetesimals through gas drag, acting
like a kind of wind," said Silsbee. "A few years ago, we realised that in
addition to the gas drag, the gravity of the disc itself dramatically alters
dynamics of the planetesimals, in some cases allowing planets to form even
despite the gravitational perturbations due to the stellar companion."
"The model we've built pulls together this work, as well as other previous
work, to test the planet formation theories," said Rafikov.
Their model found that planets can form in binary systems such as Alpha
Centauri, provided that the planetesimals start out at least 10 kilometres
across in size, and that the protoplanetary disc itself is close to
circular, without major irregularities. When these conditions are met, the
planetesimals in certain parts of the disc end up moving slowly enough
relative to each other that they stick together instead of destroying each
other.
These findings lend support to a particular mechanism of planetesimal
formation, called the streaming instability, being an integral part of the
planet formation process. This instability is a collective effect, involving
many solid particles in the presence of gas, that is capable of
concentrating pebble-to-boulder sized dust grains to produce a few large
planetesimals, which would survive most collisions.
The results of this work provide important insights for theories of planet
formation around both binary and single stars, as well as for the
hydrodynamic simulations of protoplanetary discs in binaries. In future, the
model could also be used to explain the origin of the 'Tatooine'
planets—exoplanets orbiting both components of a binary—about a dozen of
which have been identified by NASA's Kepler Space Telescope.
Reference:
K. Silsbee et al, Planet formation in stellar binaries: Global simulations
of planetesimal growth, Astronomy & Astrophysics (2021). DOI:
10.1051/0004-6361/202141139
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