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| Artist’s conception of a rocky Earth-mass exoplanet like Wolf 1069 b orbiting a red dwarf star. If the planet had retained its atmosphere, chances are high that it would feature liquid water and habitable conditions over a wide area of its dayside. Credit: NASA/Ames Research Center/Daniel Rutter |
A newly discovered exoplanet could be worth searching for signs of life.
Analyses by a team led by astronomer Diana Kossakowski of the Max Planck
Institute for Astronomy describe a planet that orbits its home star, the red
dwarf Wolf 1069, in the habitable zone.
This zone includes distances around the star for which liquid water can
exist on the surface of the planet. In addition, the planet named Wolf 1069
b has an Earth-like mass. Very likely, this planet is a rocky planet that
may also have an atmosphere. This makes the planet one of the few promising
targets to search for signs of life-friendly conditions and biosignatures.
When astronomers search for planets outside our solar system, they are
particularly interested in Earth-like planets. Of the more than 5,000
exoplanets they have discovered so far, only about a dozen have an
Earth-like mass and populate the habitable zone, the range in a planetary
system where water can maintain its liquid form on the planet's surface.
With Wolf 1069 b, the number of such exoplanets on which life could have
evolved has increased by one candidate.
A planet with eternal day and night
Detecting such low-mass planets is still a major challenge. Diana
Kossakowski and her team at the Max Planck Institute for Astronomy in
Heidelberg have taken on this task. As part of the Carmenes project, an
instrument was developed specifically for the search of potentially
habitable worlds. The Carmenes team is using this apparatus at the Calar
Alto Observatory in Spain.
"When we analyzed the data of the star Wolf 1069, we discovered a clear,
low-amplitude signal of what appears to be a planet of roughly Earth mass,"
says Diana Kossakowski. "It orbits the star within 15.6 days at a distance
equivalent to one-fifteenth of the separation between the Earth and the
sun," The results of the study have now been published in the journal
Astronomy & Astrophysics.
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| Simulated surface temperature map of Wolf 1069 b, assuming an Earth-like atmosphere. The map is centered at point that always faces the star. The temperatures are given in Kelvin. 273.15 Kelvin corresponds to zero degree Celsius. Liquid water would be possible on the planet’s surface inside the red circle. Credit: Kossakowski et al (2023) / MPIA |
According to the study, the surface of the dwarf star is relatively cool and
thus appears orange-reddish. "As a result, the so-called habitable zone is
shifted inwards," Kossakowski explains. Despite its close distance to the
central star, the planet Wolf 1069 b therefore receives only about 65% of
the incident radiant power of what Earth receives from the sun. These
special conditions make planets around red dwarf stars like Wolf 1069
potentially friendly to life.
In addition, they may all share a special property. Their rotation is
probably tidally locked to the orbit of its host star. In other words, the
star always faces the same side of the planet. So there is eternal day,
while on the other side it is always night. This is also the reason why we
always face the same side of the moon.
Climate simulations for exoplanets
If Wolf 1069 b is assumed to be a bare and rocky planet, the average
temperature even on the side facing the star would be just minus 23 degrees
Celsius. However, according to existing knowledge, it is quite possible that
Wolf 1069 b has formed an atmosphere. Under this assumption its temperature
could have increased to plus 13 degrees, as computer simulations with
climate models show. Under these circumstances, water would remain liquid
and life-friendly conditions could prevail, because life as we know it
depends on water.
An atmosphere is not only a precondition for the emergence of life from a
climatic point of view. It would also protect Wolf 1069 b from high-energy
electromagnetic radiation and particles that would destroy possible
biomolecules. The radiation and particles either stem from interstellar
space or from the central star. If the star's radiation is too intense, it
can also strip off a planet's atmosphere, as it did for Mars. But as red
dwarf, Wolf 1069 emits only relatively weak radiation.
Thus, an atmosphere may have been preserved on the newly discovered planet.
It is even possible that the planet has a magnetic field that protects it
from charged stellar wind particles. Many rocky planets have a liquid core,
which generates a magnetic field via the dynamo effect, similar to planet
Earth.
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| Illustration that compares three exoplanet systems of red dwarf stars hosting Earth-mass planets. The green rings indicate the individual habitable zones. Credit: MPIA graphics department/J. Neidel |
The difficult search for Earth-mass exoplanets
There has been immense progress in the search for exoplanets since the first
of its kind was discovered 30 years ago. Still, the signatures that
astronomers look for to detect planets with Earth-like masses and diameters
are relatively weak and hard to extract from the data. The Carmenes team is
looking for small periodic frequency shifts in the stellar spectra. These
shifts are expected to arise when a companion pulls on the host star by its
gravity, causing it to wobble. As a result. the frequency of the light
measured on Earth changes due to the Doppler effect.
In the case of Wolf 1069 and its newly discovered planet, these fluctuations
are large enough to be measured. One of the reasons is that the mass
difference between the star and planet is relatively small, causing the star
to wobble around the shared center of mass more prominently than in other
cases. From the periodic signal, the mass of the planet can be estimated, as
well.
Only a handful of candidates for future exoplanet characterization
At a distance of 31 light-years, Wolf 1069 b is the sixth closest Earth-mass
planet in the habitable zone around its host star. It belongs to a small
group of objects, such as Proxima Centauri b and Trappist-1 e, that are
candidates for biosignature searches. However, such observations are
currently beyond the capabilities of astronomical research.
"We will probably have to wait another ten years for this," Kossakowski
points out. The Extremely Large Telescope (ELT), currently under
construction in Chile, may be able to study the composition of the
atmospheres of those planets and possibly even detect molecular evidence of
life.
Reference:
D. Kossakowski et al, The CARMENES search for exoplanets around M dwarfs.
Wolf 1069 b: Earth-mass planet in the habitable zone of a nearby, very
low-mass starr, Astronomy & Astrophysics (2023).
DOI: 10.1051/0004-6361/202245322
Tags:
Space & Astrophysics