by Ravi Kumar Kopparapu
January 07, 2020
from TheConversation Website
An artist's impression
of an exoplanet in the habitable zone
around a star.
ESA/Hubble, M. Kornmesser
has captured the interest of scientists...
A few months ago a group of NASA exoplanet astronomers, who are in the business of discovering planets around other stars, called me into a secret meeting to tell me about a planet that had captured their interest.
These NASA colleagues,
...had been studying data from TESS (Transiting Exoplanet Survey Satellite) when they discovered what may be TESS' first known Earth-sized planet in a zone where liquid water could exist on the surface of a terrestrial planet.
This is very exciting news because this new planet is relatively close to Earth, and it may be possible to observe its atmosphere with either the James Webb Space Telescope (JWST) or ground-based large telescopes.
Habitable Zone Planets
There are three planets around TOI-700:
Planet d is Earth-size, within the star's habitable zone and orbits TOI-700 every 37 days.
My colleagues wanted to create a climate model for Planet d using the known properties of the star and planet. Planets b and c are Earth-size and mini-Neptune-size, respectively.
However, they orbit much closer to their host star, receiving 5 times and 2.6 times the starlight that our own Earth receives from the Sun.
For comparison, Venus, a dry and hellishly hot world with surface temperature of approximately 860 degrees Fahrenheit, receives twice the sunlight of Earth.
Until about a decade ago, only two habitable zone planets of any size were known to astronomers:
Within the last decade, however, thanks to discoveries made through both ground-based telescopes and the Kepler mission (which also looked for exoplanets from 2009 to 2019, but is now retired), astronomers have discovered about a dozen terrestrial-sized exoplanets.
These are between half and two times larger than the Earth within the habitable zones of their host stars.
Despite the relatively large number of small exoplanet discoveries to date, the majority of stars are between 600 to 3,000 light-years away from Earth - too far and dim for detailed follow-up observation.
in its star's habitable zone, the range of distances
where conditions may be just right to allow
the presence of liquid water on the surface.
Between April 2018 and now, TESS discovered more than 1,500 planet candidates.
Most are more than twice the size of Earth with orbits of less than 10 days. Earth, of course, takes 365 days to orbit around our Sun.
Liquid water is essential for habitability. It provides a medium for chemicals to interact with each other.
While it is possible for exotic life to exist at higher pressures, or hotter temperatures - like the extremophiles found near hydro-thermal vents or the microbes found half a mile beneath the West Antarctic ice sheet - those discoveries were possible because humans were able to directly probe those extreme environments.
They would not have been
detectable from space.
We know one such planet where this has already happened:
Therefore, astronomers' goal is to find those planets that are about Earth-size, orbiting at those distances from the star where water could exist in liquid form on the surface.
These planets will be our primary targets to hunt for habitable worlds and signatures of life (similar to ours) outside our solar system.
orbit a small, cool M dwarf star.
TOI 700 d is the first Earth-size habitable-zone world
discovered by TESS.
NASA's Goddard Space Flight Center
Possible Climates for Planet TOI-700 D
To prove that TOI-700 d is real, Gilbert's team needed to confirm using data from a different type of telescope.
TESS detects planets when they cross in front of the star, causing a dip in the starlight. However, such dips could also be created by other sources, such as spurious instrumental noise or binary stars in the background eclipsing each other, creating false positive signals.
Independent observations came from Joey Rodriguez at Center for Astrophysics at Harvard University.
Because we do not yet know what kind of gases this planet may actually have in its atmosphere, we use our climate models to explore possible gas combinations that would support liquid oceans on its surface.
Engelmann-Suissa, with the help of my longtime collaborator Eric Wolf, tested various scenarios including,
Based on our models, we found that if the atmosphere of planet 'TOI-700 d' contains a combination of methane or carbon dioxide or water vapor, the planet could be habitable.
Now our team needs to confirm these hypotheses with the JWST.
Bacteria living in harsh conditions
like this geothermal basin in Yellowstone National Park
provide clues about habitable zones on other planets.
If we put the same
quantity of greenhouse gases as we have on Earth on 'TOI-700 d,' the
surface temperature on this planet would still be below freezing.
One thing is for sure:
The field of exoplanets is now in a transitional era from discovering them to characterizing their atmospheres.
In the history of astronomy, new techniques enable new observations of the universe including surprises like the discovery of hot-Jupiters and mini-Neptunes, which have no equivalent in our solar system.
The stage is now set to observe the atmospheres of these planets to see which ones have conditions that support life (similar to ours)...