Earth-Like Planets Could be Right Next Door
Astronomers estimate that billions of habitable planets are orbiting red dwarf stars. What would it be like to live there?
- By Bruce Lieberman
- Air & Space magazine, June 2013
Inga Nielsen (Hamburg OBS., Gate To Nowhere)
(Page 3 of 4)
Planets that form farther out might gather up an excess of ice and, once they shimmy into the habitable zone, essentially become water worlds. “Earth’s oceans are an average of three kilometers deep if you spread them out over entire surface, but you might have an ocean [on an exoplanet] that’s 300 kilometers deep,” Kasting says. “That might be okay for marine life, but it probably would preclude the presence of continents.” Any visitors are going to need a raincoat in addition to a boat, because the planet’s highly active water cycle, Tarter writes, would produce “intense cloudiness, as well as massive precipitation.”
If, however, there’s enough water for a cloudy sky and sizable oceans, and currents strong enough to distribute heat, you could very well see our imagined exoplanet having forested continents. But even here things can get a little strange. Nancy Kiang at NASA’s Goddard Institute for Space Studies in New York has investigated the color plants might have on a habitable planet orbiting red dwarf stars, and the answer is not green.
Earth plants survive by converting the light our sun gives off into energy through photosynthesis. In most plants, the chlorophyll that enables this process absorbs blue and red light while reflecting the green light. But a red dwarf radiates much less visible light than our sun. A plant on this red dwarf planet might need to absorb as many wavelengths of light as possible to maximize photosynthesis. The plants, reflecting back almost no visible light, would appear black.
In fact, all the life on our imaginary planet might not just look different because of its red dwarf sun, it could be much, much older. Red dwarfs can exist for hundreds of billions of years before they finish the slow depletion of their fuel—indeed, they live for so long that it’s likely no red dwarf that has come into existence since the beginning of the universe has died.
For comparison, our sun is about halfway through its 10-billion-year lifespan; the simplest life on Earth developed about 3.8 billion years ago, and Homo sapiens barely more than 100,000 years ago. A red dwarf that was among the first stars that formed, nearly 13 billion years ago, could still be around today, and could theoretically be hosting a planet where life has existed for twice, maybe even three times as long as single-cell bacteria have existed on Earth.
“I’m very confident that within my lifetime—I’m 35 now—we will be able to find biosignatures of potentially habitable planets,” says Penn State’s Ravi kumar Kopparapu. His reported estimate, that as many as 61 percent of red dwarfs might have habitable planets, came from information about Venus and Mars extrapolated from our solar system’s history—that is, when water was still present on Venus before it evaporated and when the ice on Mars was liquid. Kopparapu calculated the sun’s brightness on each planet during these times and compared it to the brightness of red dwarfs. When he used this data to put limits on a habitable zone, he found it was larger than the zone Dressing used for her estimate.
Johnson, from Caltech, says speculating about what it’s like on a habitable planet orbiting a red dwarf is clearly an exercise that has just begun. “As for what’s [actually] going on on the surface of those planets, we don’t have the slightest clue, to be honest,” he says. Johnson said he’s still blown away by the thought that rocky, potentially habitable exoplanets are common. “I haven’t gotten used to that yet,” he says. “It’s not hunting anymore; it’s gathering. You just reach up into the sky and grab these things. They’re sitting right next door.”
“I would say it’s the golden age of exoplanet studies,” says Kopparapu. “The best studies are ahead, if we can get together and get missions going. Do we really want to know if we’re alone? I think we can find out, if we’re committed to it.”