Exoplanets of quiet stars most likely to sustain extraterrestrial life
For the first time, atmospheric chemistry and 3D climate modeling have been combined in a way that could help identify planets beyond our solar system which could host life. According to Northwestern University, the research could help astronomers to make progress in the search for extraterrestrial life.
"There are a lot of stars and planets out there, which means there are a lot of targets,” senior author Daniel Horton explains in a Northwestern press release. "Our study can help limit the number of places we have to point our telescopes." Space telescopes such as Hubble are able to detect water vapor and ozone on exolanets. They just need to be looking at the right ones.
The researchers focused on planets in orbit around M dwarf stars. These are the most common type of star, making up around 70 percent of those in the Milky Way. The term M dwarf roughly means red dwarf, though some definitions of red dwarf also include larger K dwarf stars.
The findings shed more light on the so-called Goldilocks zone in which life in a solar system could theoretically exist. As current understanding has it, on planets too close to their star, liquid water vaporizes. On planets too far from their star, the liquid water will freeze. It's only in between where liquid water is sustained, as is the case on Earth, that life can exist.
However, the researchers discovered that only planets orbiting stars which emit a lot of ultraviolet radiation lose water to vaporization. These are known as active stars. So-called inactive, or quiet, stars that are more likely to maintain a supply of liquid water that is all-important when it comes to being habitable.
It was only by combining the disciplines of 3D climate modeling and chemistry that the researchers discovered that a star's activity, and not merely a planet's distance from it, was the vital factor in the planet's ability to sustain life.
They also found that otherwise habitable conditions could be made dangerous if the ozone layer of the planet was too thin. In these conditions, the ultraviolet levels present would be too dangerous for any advanced life on the planet's surface.
It's nearly 60 years since Frank Drake proposed the Drake equation to quantify the number of advanced extraterrestrial civilizations in the Milky Way. It was less a serious attempt to reach an actual number than it was a speculative gambit to inspire curiosity. With the subsequent rise of exoplanetary science, the scientific search for extraterrestrial life has changed dramatically.
"For most of human history, the question of whether or not life exists elsewhere has belonged only within the philosophical realm," says study author Howard Chen in the press release. "It’s only in recent years that we have had the modeling tools and observational technology to address this question."
The team from Northwestern University worked with researchers from MIT, NASA's Virtual Planet Laboratory and the University of Colorado Boulder.
The research, titled Habitability and Spectroscopic Observability of Warm M-dwarf Exoplanets Evaluated with a 3D Chemistry-Climate Model, will be published in the Astrophysical Journal this week.
Source: Northwestern University