Model accounts for scarcity of planets with masses between super-Earths and mini-Neptunes
From: Rice News
November 7, 2022 -- A
new model that accounts for the interplay of forces acting on newborn planets
can explain two puzzling observations that have cropped up repeatedly among the
more than 3,800 planetary systems cataloged to date.
One puzzle known as the
“radius valley” refers to the rarity of exoplanets with a radius
about 1.8 times that of Earth. NASA’s Kepler spacecraft observed planets of
this size about 2-3 times less frequently than it observed super-Earths with
radii about 1.4 times that of Earth and mini-Neptunes with radii about 2.5
times Earth’s. The second mystery, known as “peas in a pod,” refers to neighboring
planets of similar size that have been found in hundreds of planetary systems.
Those include TRAPPIST-1 and Kepler-223, which also feature
planetary orbits of near-musical harmony.
“I believe we are the
first to explain the radius valley using a model of planet formation and
dynamical evolution that self-consistently accounts for multiple constraints of
observations,” said Rice University’s André Izidoro, corresponding author
of a study published this week in Astrophysical Journal Letters.
“We’re also able to show that a planet-formation model incorporating giant
impacts is consistent with the peas-in-a-pod feature of exoplanets.”
Izidoro, a Welch
Postdoctoral Fellow at Rice’s NASA-funded CLEVER
Planets project, and co-authors used a supercomputer to simulate the first
50 million years of the development of planetary systems using a planetary
migration model. In the model, protoplanetary disks of gas and dust that give
rise to young planets also interact with them, pulling them closer to their
parent stars and locking them in resonant orbital chains. The chains are broken
within a few million years, when the disappearance of the protoplanetary disk
causes orbital instabilities that lead two or more planets to slam into one
another.
Planetary migration
models have been used to study planetary systems that have retained their
resonant orbital chains. For example, Izidoro and CLEVER Planets
colleagues used a migration model in 2021 to calculate the maximum
amount of disruption TRAPPIST-1’s seven-planet system could have withstood
during bombardment and still retained its harmonious orbital structure.
In the new study,
Izidoro partnered with CLEVER Planets’ investigators Rajdeep Dasgupta and Andrea
Isella, both of Rice, Hilke Schlichting of the University of
California, Los Angeles, and Christian Zimmermann and Bertram Bitsch of the Max
Planck Institute for Astronomy in Heidelberg, Germany.
“The migration of young
planets towards their host stars creates overcrowding and frequently results in
cataclysmic collisions that strip planets of their hydrogen-rich atmospheres,”
Izidoro said. “That means giant impacts, like the one that formed our moon, are
probably a generic outcome of planet formation.”
The research suggests
planets come in two “flavors,” super-Earths that are dry, rocky and 50% larger
than Earth, and mini-Neptunes that are rich in water ice and about 2.5 times
larger than Earth. Izidoro said new observations seem to support the results,
which conflict with the traditional view that both super-Earths and
mini-Neptunes are exclusively dry and rocky worlds.
Based on their
findings, the researchers made predictions that can be tested by NASA’s James
Webb Space Telescope. They suggest, for instance, that a fraction of planets
about twice Earth’s size will both retain their primordial hydrogen-rich
atmosphere and be rich in water.
The research was funded
by NASA (80NSSC18K0828), the Welch Foundation (C-2035-20200401) and the
European Research Council (757448-PAMDORA).
https://news.rice.edu/news/2022/early-planetary-migration-can-explain-missing-planets
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