Ocean world: Rocky exoplanet has just half the mass of Venus

Date:
August 5, 2021
Source:
ESO
Summary:
A team of astronomers have shed new light on planets around a
nearby star, L 98-59, that resemble those in the inner Solar
System. Amongst the findings are a planet with half the mass of
Venus -- the lightest exoplanet ever to be measured using the
radial velocity technique -- an ocean world, and a possible planet
in the habitable zone.



FULL STORY ==========================================================================
A team of astronomers have used the European Southern Observatory's
Very Large Telescope (ESO's VLT) in Chile to shed new light on planets
around a nearby star, L 98-59, that resemble those in the inner Solar
System. Amongst the findings are a planet with half the mass of Venus --
the lightest exoplanet ever to be measured using the radial velocity
technique -- an ocean world, and a possible planet in the habitable zone.


==========================================================================
"The planet in the habitable zone may have an atmosphere that could
protect and support life," says Mari'a Rosa Zapatero Osorio, an astronomer
at the Centre for Astrobiology in Madrid, Spain, and one of the authors
of the study published today in Astronomy & Astrophysics.

The results are an important step in the quest to find life on Earth-sized planets outside the Solar System. The detection of biosignatures on an exoplanet depends on the ability to study its atmosphere, but current telescopes are not large enough to achieve the resolution needed to do
this for small, rocky planets. The newly studied planetary system, called
L 98-59 after its star, is an attractive target for future observations
of exoplanet atmospheres. Its orbits a star only 35 light-years away
and has now been found to host rocky planets, like Earth or Venus,
which are close enough to the star to be warm.

With the contribution of ESO's VLT, the team was able to infer that three
of the planets may contain water in their interiors or atmospheres. The
two planets closest to the star in the L 98-59 system are probably dry,
but might have small amounts of water, while up to 30% of the third
planet's mass could be water, making it an ocean world.

Furthermore, the team found "hidden" exoplanets that had not previously
been spotted in this planetary system. They discovered a fourth planet
and suspect there is a fifth, in a zone at the right distance from the
star for liquid water to exist on its surface. "We have hints of the
presence of a terrestrial planet in the habitable zone of this system," explains Olivier Demangeon, a researcher at the Instituto de Astrofi'sica
e Cie^ncias do Espac,o, University of Porto in Portugal and lead author
of the new study.

The study represents a technical breakthrough, as astronomers were able
to determine, using the radial velocity method, that the innermost planet
in the system has just half the mass of Venus. This makes it the lightest exoplanet ever measured using this technique, which calculates the wobble
of the star caused by the tiny gravitational tug of its orbiting planets.

The team used the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) instrument on ESO's VLT to study
L 98-59.

"Without the precision and stability provided by ESPRESSO this
measurement would have not been possible," says Zapatero Osorio. "This
is a step forward in our ability to measure the masses of the smallest
planets beyond the Solar System." The astronomers first spotted three
of L 98-59's planets in 2019, using NASA's Transiting Exoplanet Survey Satellite (TESS). This satellite relies on a technique called the transit method -- where the dip in the light coming from the star caused by a
planet passing in front of it is used to infer the properties of the
planet -- to find the planets and measure their sizes.

However, it was only with the addition of radial velocity measurements
made with ESPRESSO and its predecessor, the High Accuracy Radial velocity Planet Searcher (HARPS) at the ESO La Silla 3.6-metre telescope, that
Demangeon and his team were able to find extra planets and measure the
masses and radii of the first three. "If we want to know what a planet
is made of, the minimum that we need is its mass and its radius,"
Demangeon explains.

The team hopes to continue to study the system with the forthcoming
NASA/ESA/ CSA James Webb Space Telescope (JWST) , while ESO's Extremely
Large Telescope (ELT), under construction in the Chilean Atacama Desert
and set to start observations in 2027, will also be ideal for studying
these planets. "The HIRES instrument on the ELT may have the power to
study the atmospheres of some of the planets in the L 98-59 system,
thus complementing the JWST from the ground," says Zapatero Osorio.

"This system announces what is to come," adds Demangeon. "We,
as a society, have been chasing terrestrial planets since
the birth of astronomy and now we are finally getting closer
and closer to the detection of a terrestrial planet in the
habitable zone of its star, of which we could study the atmosphere." ========================================================================== Story Source: Materials provided by ESO. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. O. D. S. Demangeon, M. R. Zapatero Osorio, Y. Alibert,
S. C. C. Barros,
V. Adibekyan, H. M. Tabernero, A. Antoniadis-Karnavas,
J. D. Camacho, et al. Warm terrestrial planet with half the mass
of Venus transiting a nearby star. Astronomy & Astrophysics, 2021;
DOI: 10.1051/0004-6361/ 202140728 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210805090842.htm

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