We will need to be more selective when choosing the targets for our search
for indications of life because the number of verified exoplanets in the
Milky Way is increasing.
The possibilities have been reduced by a group of astronomers led by Anna
Shapiro of the Max Planck Institute for Solar System Studies in
Germany.
A recent research suggests that Earth-like exoplanets with relatively
little metal content that orbit Sun-like stars are more likely to be
shielded from damaging UV radiation that poses a risk of genetic damage to
life.
Since that stars with a lower metal composition radiate more ultraviolet
radiation, this may appear counterintuitive. Yet, the team's research
demonstrates that an oxygen-rich planet has a stronger ozone layer than one
with a metal-rich host, providing a world orbiting a metal-poor star with
more protection.
Our results suggest that the ideal places to look for sophisticated life on
land are planets inhabited by stars with low metallicity, the
authors wrote in their study.
Not every star is made equally. They might be little, chilly, and gloomy or
big, blazing, and scorching. Even while they share certain fundamental
components, their chemical make-up might differ greatly.
This is due to the absence of heavy elements at the beginning of the
universe's history. There was essentially only hydrogen and helium; from
these components, the earliest stars were formed, their centers being
enormous motors that crashed smaller, lighter atoms together.
These stars' violent deaths produced even heavier elements, which were then
ejected and seeded into space where they were absorbed by newborn stars
formed from clouds of interstellar dust and gas.
The star's radiation emission is altered by these components. Higher
metallicity, or stars with more elements heavier than hydrogen and helium,
produce less ultraviolet radiation than stars formed of lighter materials.
And because humans have spent our whole lives on Earth, we are aware that UV
radiation may damage DNA
in many ways in sensitive land-dwelling species.
Shapiro and her coworkers looked at how UV irradiation would affect whether
or not an alien world might support life by using Earth as a model.
It's possible that an extraterrestrial civilisation observing the Solar
System from a considerable distance might find Earth to be uninhabitable.
According to the researchers, UV-C and UV-B radiation levels are "far over
the maximum tolerated range for terrestrial life" at our present distance
from the Sun.
Yet most of it is blocked by our atmosphere, which has layers of oxygen
(O2) in the upper atmosphere, which absorbs most UV-C, and ozone (O3) in the
center, which absorbs UV-B.
Ozone is produced and destroyed by ultraviolet light. O2 molecules can be
fragmented at wavelengths below 240 nanometers, at which point floating O
atoms may meet and bond with O2 molecules to create O3. Yet longer
wavelengths cause the O3 to dissociate by
photodissociation. Then, the produced O atoms might unite again to form O2.
The metallicity and temperature of a star, along with other parameters, all
affect the UV radiation of that star. Shapiro and her colleagues created
computer models of planets similar to Earth orbiting stars similar to the
Sun, adjusting the settings to determine how UV radiation would affect the
orbiting exoplanet.
They discovered that, contrary to what may be expected, metallicity has a
greater impact on the habitability of the exoplanet than temperature. The
likelihood of habitable planets was higher for stars with lower
metallicities and higher UV radiation.
That's because less UV light reached the exoplanet's surface as a result of
how the UV radiation interacted with the oxygen in the atmosphere, which
improved the shield.
The associated stellar radiative spectrum in oxygenated planetary
atmospheres allows less O3 formation, which enhances UV penetration, making
the conditions on planets orbiting these stars less hospitable for the
biosphere on land. This is paradoxical because higher metallicity stars,
which have
emerged later
in the history of the Universe, emit less UV radiation.
"Therefore, we discover that the UV radiation exposure on the surfaces of
planets around metal-rich stars is greater than that on planets orbiting
metal-poor stars. The greatest places to look for complicated life on earth
are planets in the habitable zones of stars with low metallicities.
It's not sufficient to completely rule out stars with greater metallicities
just yet. Yet the classification and study of exoplanet atmospheres using
tools like the James Webb Space Telescope will assist researchers in
determining whether their discoveries are on the correct track, putting us a
small bit closer to discovering evidence of life on an extraterrestrial
planet.
The research has been published in
Nature Communications.
