Image: Chester Herman
One of the most important characteristics of an alien planet is whether or not it falls into what's called the habitable zone — a Goldilocks-like range of not-too-close, not-too-far distances from the parent star that might allow the planet to host life.
Now scientists have redefined the boundaries of the habitable zone for alien planets, potentially kicking out some exoplanaets that were thought to fall within it, and maybe allowing a few that had been excluded to squeeze in.
"This will have a significant impact on the number of exoplanets that are within habitable zone," said research team leader Ravi Kumar Kopparapu of Penn State University.
The habitable zone defines the region where a planet might be able to retain liquid water on its surface. Any closer to the star and water would vaporize away; any farther, and it would freeze to ice. But water in its liquid state is what scientists are after, since that is thought to be a prerequisite for life.
The new definition of the habitable zone is based on updated atmospheric databases called HITRAN (high-resolution transmission molecular absorption) and HITEMP (high-temperature spectroscopic absorption parameters), which give the absorption parameters of water and carbon dioxide — two properties that strongly influence the atmospheres of exoplanets, determining whether those planets could host liquid water. [9 Exoplanets That Could Host Alien Life]
The scientists cautioned that the habitable zone definition still does not take into account feedback effects from clouds, which will also affect a planet's habitability.
The previous habitable zone definitions were derived about 20 years ago by Penn State researcher James Kasting, who was also part of the team behind the updates.
"At the time when he wrote that paper no exoplanets were discovered," Kopparapu told SPACE.com. "In 20 years, hundreds, maybe thousands have been discovered."
The new definition isn't radically different from the old one. For example, in our own solar system, the boundaries of the habitable zone have shifted from between 0.95 astronomical units (AU, or the distance between Earth and the sun) and 1.67 AU, to the new range of 0.99 AU to 1.7 AU.
"It's a surprise that Earth is so close to the inner edge of the habitable zone," said astronomer Abel Méndez of the University of Puerto Rico at Arecibo, who was not part of the team behind the redefinition.
Méndez manages a list, called the Habitable Exoplanet Catalog, off all the known planets beyond our solar system that could be habitable to life. The new study will necessitate some adjustments to the catalog, he said.
"Right now as I see it as a significant change," Méndez said. "Many of those planets that we believe were inside are now outside. But on the other side, it extends the habitable zone's outer edge, so a few planets that are farther away might fall inside the habitable zone now."
He mentioned one planet in particular, Gliese 581d, was thought to lie at the outer edge of its star's habitable zone. With the new definition, though, it falls almost smack in the middle, making it perhaps a better candidate for extraterrestrial life.
"That will be a big change for that particular planet," Méndez said. "That means the prospects for life on the planet will be much better."
The researchers detail their new habitable zone definition in a paper to be published in an upcoming issue of the Astrophysical Journal.
To explore the Habitable Planet Catalog directy, visit: http://phl.upr.edu/projects/habitable-exoplanets-catalog
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6 Comments
Add Comment"It's a surprise that Earth is so close to the inner edge of the habitable zone."
Reply | Report Abuse | Link to thisIt isn't a surprise for those of us who recognize how the lingering effects of the Great Ice-age have distorted our concept of planet habitability versus position. The truth of the matter is that our planet, when climate normalized, isn't quite as hospitable for human life as we have learned to enjoy.
Something is off. Moving the Earth just over 1% closer to the Sun will put us out of the habitable zone? I realize that the amount of solar radiation would increase by a larger percent.
Reply | Report Abuse | Link to thisThe Great Ice-Age is not over, if you look into the subject there are periods where the tempurature increases to what it is like today. I believe since recorded history we have been in one of those periods.
Reply | Report Abuse | Link to thisIt hasn't been that long since I read an article describing the habitable zone as an area including Venus, Earth, and Mars. Mars' small size and lack of a magnetic field have cost it atmosphere and water, and Venus has overheated due to it's massive atmosphere, but both could be much more Earthlike with some terraforming. Has all this changed, or just the definition?
Reply | Report Abuse | Link to thisDoesn't it depend on the partial pressures (atmospheric density) of such greenhouse gases as water and carbon dioxide? How can we measure that from transits across distant stars? And what about subterranean oceans as on Ganymede? Tidal heat allows a "goldilocks zone" farther than 1.7 AU from the sun.
Reply | Report Abuse | Link to thisIt is still early days yet. Give it about 50 years and we will have a more meaningful "goldilocks zone" definition. There are a lot of additional factors such as gravitational pull at the surface. The greater the gravity strength, the denser the atmosphere. Then there is atmospheric composition (or the lack of an atmosphere).
Reply | Report Abuse | Link to thisDistance from a star is only the first building block of the foundation.