Scientists discover rare six-planet system that moves in strange synchrony
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An artist's illustration of the six newly discovered planets circling
their star in resonance. Image by Roger Thibaut (NCCR PlanetS). Credit: Roger
Thibaut (NCCR PlanetS)
A study led led by UChicago astronomer Rafael Luque may tell us about how planets form.
Scientists have discovered a rare sight in a nearby
star system: Six planets orbiting their central star in a rhythmic beat. The
planets move in an orbital waltz that repeats itself so precisely that it can
be readily set to music.
A rare case of an "in sync"
gravitational lockstep, the system could offer deep insight into planet formation and evolution.
The analysis, led by UChicago scientist
Rafael Luque, was published Nov. 29 in Nature.
"This discovery is going to become
a benchmark system to study how sub-Neptunes, the most
common type of planets outside of the solar system, form, evolve, what are they made of, and if they
possess the right conditions to support the existence of liquid water in their
surfaces," said Luque.
A rare
resonance
The six planets orbit a
star known as HD110067,
which lies around 100 light-years away in the northern constellation of Coma
Berenices.
In 2020, NASA's Transiting Exoplanet
Survey Satellite (TESS)
detected dips in the star's brightness that indicated planets were passing in
front of the star's surface. Combining data from both TESS and the European
Space Agency's CHaracterizing ExOPlanet Satellite (Cheops), a team of researchers
analyzed the data and discovered a first-of-its-kind configuration.
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A rare family of six exoplanets has been unlocked with the help of ESA’s
Cheops mission. The planets in this family are all smaller than Neptune and
revolve around their star HD110067 in a very precise waltz. When the closest
planet to the star makes three full revolutions around it, the second one makes
exactly two during the same time. This is called a 3:2 resonance. The six
planets form a resonant chain in pairs of 3:2, 3:2, 3:2, 4:3, and 4:3,
resulting in the closest planet completing six orbits while the outer-most
planet does one. Cheops confirmed the orbital period of the third planet in the
system, which was the key to unlocking the rhythm of the entire system. This is
the second planetary system in orbital resonance that Cheops has helped to
reveal. The first one is called TOI-178. Credit: ESA.
While multi-planet systems are common in
our galaxy, those in a tight gravitational formation known as
"resonance" are observed by astronomers far less often.
In this case, the planet closest to the
star makes three orbits for every two of the next planet out—called a 3/2
resonance—a pattern that is repeated among the four closest planets. Among the
outermost planets, a pattern of four orbits for every three of the next planet
out (a 4/3 resonance) is repeated twice.
And these resonant orbits are
rock-solid: The planets likely have been performing this same rhythmic dance
since the system formed billions of years ago, the scientists said.
Formation
stories
Orbitally resonant systems are extremely
important to find because they tell astronomers about the formation and
subsequent evolution of the planetary system. Planets around stars tend to form
in resonance but can be easily perturbed. For example, a very massive planet, a
close encounter with a passing star, or a giant impact event can all disrupt
the careful balance.
As a result, many of the multi-planet
systems known to astronomers are not in resonance but look close enough that
they could have been resonant once. However, multi-planet systems preserving
their resonance are rare.
"We think only about one percent of
all systems stay in resonance, and even fewer show a chain of planets in such
configuration," said Luque. That why HD110067 is special and invites
further study: "It shows us the pristine configuration of a planetary
system that has survived untouched."
More precise measurements of these planets' masses and orbits will be needed to further sharpen
the picture of how the system formed.
