from
https://www.cnn.com/2023/11/29/world/six-exoplanets-resonance-scn/index.html

Astronomers discover nearby six-planet solar system with ‘pristine
configuration’
Ashley Strickland
By Ashley Strickland, CNN
6 minute read
Updated 7:35 PM EST, Wed November 29, 2023

Tracing a link between two neighbour planet at regular time interval
along their orbits, create a pattern unique to each couple. The six
planets of the HD110067 system create together a mesmerising geometric
pattern due to their resonance-chain.
The orbits of the six planets revolving around a star called HD110067
create a geometric pattern due to their resonance.
Thibaut Roger/NCCR PlanetS
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CNN
—
Astronomers have used two different exoplanet-detecting satellites to
solve a cosmic mystery and reveal a rare family of six planets located
about 100 light-years from Earth. The discovery could help scientists
unlock the secrets of planet formation.

The six exoplanets orbit a bright star similar to the sun named
HD110067, which is located in the Coma Berenices constellation in the
northern sky. Larger than Earth but smaller than Neptune, the planets
are in a little-understood class called sub-Neptunes commonly found
orbiting sunlike stars in the Milky Way. And the planets, labeled b
through g, revolve around the star in a celestial dance known as orbital
resonance.

There are discernible patterns as the planets complete their orbits and
exert gravitational forces on one another, according to a study
published Wednesday in the journal Nature. For every six orbits
completed by planet b, the closest planet to the star, the outermost
planet g completes one.

As planet c makes three revolutions around the star, planet d does two,
and when planet e completes four orbits, planet f does three.

This harmonic rhythm creates a resonant chain, with all six planets
aligning every few orbits.

What makes this planetary family an unusual find is that little has
changed since the system formed more than 1 billion years ago, and the
revelation could shed light on the evolution of planets and the origin
of prevalent sub-Neptunes in our home galaxy.

Detecting a mystery
Researchers first took notice of the star system in 2020 when NASA’s
Transiting Exoplanet Survey Satellite, or TESS, detected dips in the
brightness of HD110067. A dip in starlight often suggests the presence
of a planet that’s passing between its host star and an observing
satellite as the planet travels along its orbital path. Detecting these
dips in luminosity, known as the transit method, is one of the main
strategies used by scientists to identify exoplanets via ground and
space-based telescopes.

Astronomers determined the orbital periods of two planets around the
star from that 2020 data. Two years later, TESS observed the star again,
and the evidence suggested different orbital periods for those planets.

When the data sets didn’t add up, astronomer and lead study author
Rafael Luque and some of his colleagues decided to take another look at
the star using a different satellite — the European Space Agency’s
CHaracterising ExOPlanet Satellite, or Cheops. While TESS is used to
observe fractions of the night sky for short observations, Cheops
observes one star at a time.

Artist's impression of CHEOPS.
This artist's illustration shows Cheops in orbit around Earth as it
searches for exoplanets.
ESA/ATG medialab
“We went fishing for signals among all the potential periods that those
planets could have,” said Luque, a postdoctoral scholar in the
University of Chicago’s department of astronomy and astrophysics.

The data collected by Cheops helped the team solve the “detective story”
started by TESS, he said. Cheops was able to determine the presence of a
third planet in the system, which was crucial to confirming the orbital
periods of the other two planets, as well as their rhythmic resonance.

As the team matched up the rest of the unexplained TESS data with the
Cheops observations, they discovered the other three planets orbiting
the star. Follow-up observations with ground-based telescopes confirmed
the presence of the planets.

The dedicated time Cheops spent observing the star helped astronomers
iron out the mixed signals from the TESS data to determine how many
planets were crossing in front of the star and the resonance of their
orbits.

“Cheops gave us this resonant configuration that allowed us to predict
all the other periods. Without that detection from Cheops, it would have
been impossible,” Luque said.

The closest planet takes just over nine Earth days to complete an orbit
around the star, and the most distant takes about 55 days. All of the
planets have quicker revolutions around their star than Mercury, which
takes 88 days to complete one lap around the sun.

Given how close they are to HD110067, the planets likely have blistering
average temperatures similar to Mercury and Venus, ranging between 332
degrees Fahrenheit and 980 degrees Fahrenheit (167 degrees Celsius and
527 degrees Celsius).

Why planetary rhythm matters
The formation of planetary systems, like our own solar system, can be a
violent process. While astronomers believe that planets tend to
initially form in resonance around stars, the gravitational influence of
massive planets, a graze with a passing star or a collision with another
celestial body can upset the harmonic balance.

Most planetary systems are not in resonance, and those with multiple
planets that have preserved their initial rhythmic orbits are rare,
which is why astronomers want to study HD110067 and its planets as a
“rare fossil” in detail, Luque said.

This artist's concept shows what the exoplanet WASP-17 b could look like.

WASP-17 b, also called Ditsö̀, is a hot gas giant that orbits its star
at a distance of just 0.051 AU (about 4.75 million miles, or one-eighth
the distance between Mercury and the Sun), completing one full circuit
in about 3.7 Earth-days. The system lies within the Milky Way, about
1,300 light-years from Earth, in the constellation Scorpius.

With a volume more than seven times that of Jupiter and a mass less than
one-half of Jupiter, WASP-17 b is an extremely puffy planet. Its short
orbital period, large size, and thick, extended atmosphere make it ideal
for observation using transmission spectroscopy, which involves
measuring the effects of the planet's atmosphere on the starlight
filtering through it.

WASP-17 b's atmosphere is composed primarily of hydrogen and helium,
along with small amounts of water vapor and hints of carbon dioxide and
other molecules. Observations of 5- to 12-micron infrared light from
Webb's MIRI (Mid-Infrared Instrument) show that WASP-17 b's atmosphere
also contains clouds made of nanocrystals of quartz (SiO2).

WASP-17 b is tidally locked and has a retrograde orbit. Its temperature
ranges from about 1,000 kelvins (1,350 degrees F or 725 degrees C) on
the cooler nightside to nearly 2,000 kelvins (3,150 degrees F or 1,725
degrees C) on the side in permanent daylight.

The star, WASP-17 (also called Diwö), is an F-type star: slightly
larger, more massive, hotter, and whiter than the Sun.

This artist's concept is based on new data gathered by MIRI as well as
previous observations from other ground- and space-based telescopes,
including NASA's Hubble and retired Spitzer space telescopes. Webb has
not captured any images of the planet.
Quartz crystals detected swirling in an exoplanet’s atmosphere
“We think only about one percent of all systems stay in resonance,”
Luque said in a statement. “It shows us the pristine configuration of a
planetary system that has survived untouched.”

The discovery is the second time Cheops has helped reveal a planetary
system with orbital resonance. The first one, known as TOI-178, was
announced in 2021.

“As our science team puts it: Cheops is making outstanding discoveries
sound ordinary. Out of only three known six-planet resonant systems,
this is now the second one found by Cheops, and in only three years of
operations,” said Maximilian Günther, ESA Cheops project scientist, in a
statement.

A perfect observation target
The system can also be used to study how sub-Neptunes form, the study
authors said.

While sub-Neptunes are common in the Milky Way galaxy, they don’t exist
in our own solar system. And there is little agreement among astronomers
about how these planets form and what they’re made of — so an entire
system consisting of sub-Neptunes could help scientists determine more
about their origin, Luque said.

Many exoplanets have been found orbiting dwarf stars that are much
cooler and smaller than our sun, such as the famed TRAPPIST-1 system and
its seven planets, announced in 2017. While the TRAPPIST-1 system also
has a resonant chain, the faintness of the host star makes observations
difficult.

exoplanet K2-18 b illustration
Planet in ‘habitable’ zone could have rare oceans and a possible sign of
life, Webb data reveals
But HD110067, which has 80% the mass of our sun, is the brightest known
star with more than four planets in orbit, so observing the system is
much easier.

Initial detections of the mass of the planets suggest that some of them
have puffy hydrogen-rich atmospheres, which makes them ideal targets of
study for the James Webb Space Telescope. As starlight filters through
the planets’ atmospheres, Webb can be used to determine the composition
of each world.

“The sub-Neptune planets of the HD110067 system appear to have low
masses, suggesting they may be gas- or water-rich. Future observations,
for example with the James Webb Space Telescope, of these planetary
atmospheres could determine whether the planets have rocky or water-rich
interior structures,” said study coauthor Jo Ann Egger, doctoral student
in astrophysics at the University of Bern in Switzerla