Elusive planets play 'hide and seek' with CHEOPS

Date:
June 8, 2023
Source:
University of Bern
Summary:
Astronomers have clearly identified the existence of four new
exoplanets.

The four mini-Neptunes are smaller and cooler, and more difficult
to find than the so-called Hot Jupiter exoplanets which have been
found in abundance.


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FULL STORY ==========================================================================
With the help of the CHEOPS space telescope an international team of
European astronomers managed to clearly identify the existence of four
new exoplanets.

The four mini-Neptunes are smaller and cooler, and more difficult to
find than the so-called Hot Jupiter exoplanets which have been found
in abundance. Two of the four resulting papers are led by researchers
from the University of Bern and the University of Geneva who are also
members of the National Centre of Competence in Research (NCCR) PlanetS.

CHEOPS is a joint mission by the European Space Agency (ESA)
and Switzerland, under the leadership of the University of Bern in collaboration with the University of Geneva. Since its launch in December
2019, the extremely precise measurements of CHEOPS have contributed to
several key discoveries in the field of exoplanets.

NCCR PlanetS members Dr. Sole`ne Ulmer-Moll of the Universities of Bern
and Geneva, and Dr. Hugh Osborn of the University of Bern, exploited the
unique synergy of CHEOPS and the NASA satellite TESS, in order to detect
a series of elusive exoplanets. The planets, called TOI 5678 b and HIP
9618 c respectively, are the size of Neptune or slightly smaller with
4.9 and 3.4 Earth radii. The respective papers have just been published
in the journals Astronomy & AstrophysicsandMonthly Notices of the Royal Astronomical Society. Publishing in the same journals,two other members of
the international team, Amy Tuson from the University of Cambridge (UK)
and Dr. Zolta'n Garai from the ELTE Gothard Astrophysical Observatory (Hungary), used the same technique to identify two similar planets in
other systems.

The synergy of two satellites The CHEOPS satellite observes the luminosity
of stars in order to capture the slight dimming that occurs when, and
if, an orbiting planet happens to pass in front of its star from our
point of view. By searching for these dimming events, called "transits," scientists have been able to discover the majority of the thousands of exoplanets known to orbit stars other than our Sun.

"NASA's TESS satellite excels at detecting the transits of exoplanets,
even for the most challenging small planets. However, it changes its
field of view every 27 days in order to scan rapidly most of the sky,
which prevents it from finding planets on longer orbital periods,"
explains Hugh Osborn. Still, the TESS satellite was able to observe
single transits around the stars TOI 5678 and HIP 9618. When returning
to the same field of view after two years, it could again observe similar transits around the same stars. Despite these observations, it was still
not possible to conclude unequivocally to the presence of planets around
those stars as information was incomplete.

"This is where CHEOPS comes into play: Focusing on a single-star at
a time, CHEOPS is a follow-up mission which is perfect to continue
observing these stars to find the missing bits of information,"
complements Sole`ne Ulmer-Moll.

A lengthy game of "hide and seek" Suspecting the presence of exoplanets,
the CHEOPS team designed a method to avoid spending blindly precious
observing time in the hope to detect additional transits. They adopted
a targeted approach based on the very few clues the transits observed by
TESS provided. Based on this, Osborn developed a software which proposes
and prioritizes candidate periods for each planet. "We then play a sort
of 'hide and seek' game with the planets, using the CHEOPS satellite,"
as Osborn says.

"We point CHEOPS towards a target at a given time, and depending if we
observe a transit or not, we can eliminate some of the possibilities
and try again at another time until there is a unique solution for the
orbital period." It took five and four attempts respectively for the
scientists to clearly confirm the existence of the two exoplanets and
determine that TOI 5678 b has a period of 48 days, while HIP 9618 c has
a period of 52.5 days.

Ideal targets for the JWST The story does not end there for the
scientists. With the newly found constrained periods, they could turn to ground-based observations using another technique called radial velocity,
which enabled the team to determine masses of respectively 20 and 7.5
Earth masses for TOI 5678 b and HIP 9618 c. With both the size and mass
of a planet, its density is known, and scientists can get an idea of what
it is made off. "For mini-Neptunes however, density is not enough, and
there are still a few hypotheses as for the composition of the planets:
they could either be rocky planets with a lot of gas, or planets rich in
water and with a very steamy atmosphere," explains Ulmer-Moll. "Since the
four newly discovered exoplanets are orbiting bright stars, it also makes
them targets of prime interest for the mission of the James Webb Space Telescope JWST which might help to solve the riddle of their composition," Ulmer-Moll continues.

Most exoplanets atmospheres observed so far have been from Hot Jupiters,
which are very big and hot exoplanets orbiting close to their parent
star. "The four new planets which we detected have much more moderate temperatures of 'only' 217 to 277-oC. These temperatures enable clouds
and molecules to survive, which would otherwise be destroyed by the
intense heat of Hot Jupiters. And they may potentially be detected by
the JWST," as Osborn explains. Smaller in size and with a longer orbital
period than Hot Jupiters, the four newly detected planets are a first
step towards the observation of transiting Earth-like planets.

* RELATED_TOPICS
o Space_&_Time
# Extrasolar_Planets # Satellites # Pluto # Stars #
Kuiper_Belt # NASA # Astronomy # Space_Exploration
* RELATED_TERMS
o Extrasolar_planet o Jupiter o Jupiter's_moons o Asteroid o
Saturn o Dysnomia_(moon_of_Eris) o Uranus'_natural_satellites
o Astronomy

========================================================================== Story Source: Materials provided by University_of_Bern. Note: Content
may be edited for style and length.


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Society, 2023; 523 (2): 3090 DOI: 10.1093/mnras/stad1369 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/06/230608120947.htm

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