When the world's most powerful telescope launches into space this year,
scientists will learn whether Earth-sized planets in our 'solar
neighborhood' have a key prerequisite for life—an atmosphere.
These planets orbit an M-dwarf, the smallest and most common type of star in
the galaxy. Scientists do not currently know how common it is for Earth-like
planets around this type of star to have characteristics that would make
them habitable.
"As a starting place, it is important to know whether small, rocky planets
orbiting M-dwarfs have atmospheres," said Daria Pidhorodetska, a doctoral
student in UC Riverside's Department of Earth and Planetary Sciences. "If
so, it opens up our search for life outside our solar system."
To help fill this gap in understanding, Pidhorodetska and her team studied
whether the soon-to-launch James Webb Space Telescope, or the
currently-in-orbit Hubble Space Telescope, are capable of detecting
atmospheres on these planets. They also modeled the types of atmospheres
likely to be found, if they exist, and how they could be distinguished from
each other. The study has now been published in the Astronomical Journal.
Study co-authors include astrobiologists Edward Schwieterman and Stephen
Kane from UCR, as well as scientists from Johns Hopkins University, NASA's
Goddard Space Flight Center, Cornell University and the University of
Chicago.
The star at the center of the study is an M-dwarf called L 98-59, which
measures only 8% of our sun's mass. Though small, it is only 35 light years
from Earth. It's brightness and relative closeness make it an ideal target
for observation.
Shortly after they form, M-dwarfs go through a phase in which they can shine
two orders of magnitude brighter than normal. Strong ultraviolet radiation
during this phase has the potential to dry out their orbiting planets,
evaporating any water from the surface and destroying many gases in the
atmosphere.
"We wanted to know if the ablation was complete in the case of the two rocky
planets, or if those terrestrial worlds were able to replenish their
atmospheres," Pidhorodetska said.
The researchers modeled four different atmospheric scenarios: one in which
the L 98-59 worlds are dominated by water, one in which the atmosphere is
mainly composed of hydrogen, a Venus-like carbon dioxide atmosphere, and one
in which the hydrogen in the atmosphere escaped into space, leaving behind
only oxygen and ozone.
They found that the two telescopes could offer complementary information
using transit observations, which measure a dip in light that occurs as a
planet passes in front of its star. The L 98-59 planets are much closer to
their star than Earth is to the sun. They complete their orbits in less than
a week, making transit observations by telescope faster and more cost
effective than observing other systems in which the planets are farther from
their stars.
"It would only take a few transits with Hubble to detect or rule out a
hydrogen- or steam-dominated atmosphere without clouds," Schwieterman said.
"With as few as 20 transits, Webb would allow us to characterize gases in
heavy carbon dioxide or oxygen-dominated atmospheres."
Of the four atmospheric scenarios the researchers considered, Pidhorodetska
said the dried-out oxygen-dominated atmosphere is the most likely.
"The amount of radiation these planets are getting at that distance from the
star is intense," she said.
Though they may not have atmospheres that lend themselves to life today,
these planets can offer an important glimpse into what might happen to Earth
under different conditions, and what might be possible on Earth-like worlds
elsewhere in the galaxy.
The L 98-59 system was only discovered in 2019, and Pidhorodetska said she
is excited to get more information about it when Webb is launched later this
year.
"We're on the precipice of revealing the secrets of a star system that was
hidden until very recently," Pidhorodetska said.
Reference:
Daria Pidhorodetska et al, L 98-59: A Benchmark System of Small Planets for
Future Atmospheric Characterization, The Astronomical Journal (2021).
DOI: 10.3847/1538-3881/ac1171
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