As we continue the hunt for habitable worlds beyond our solar system, we're finding more and more candidates closer to home. There's even a small rocky exoplanet within the so-called "habitable zone" at Proxima Centauri, the dinky red dwarf star right next door. But there's more, and astronomers are beginning to identify which of these strange new worlds we could soon get a good look at with the next generation of advanced telescopes on Earth and in space.
One tantalizing potentially habitable exoplanet orbits the star Wolf 1061, only 14 light-years away - a distance that is practically on our galactic doorstep. Known to host three exoplanets, the Wolf 1061 system is interesting as it could be a target for NASA's James Webb Space Telescope (JWST) that is scheduled to launch in 2018. Sitting at the sun-Earth L2 point - an island of gravitational calm nearly one million miles away in Earth's shadow - the infrared JWST could be used to detect atmospheric components in worlds that could, hypothetically, support life. Other exoplanet-hunting projects are being launched, such as the Transiting Exoplanet Survey Satellite (TESS), the CHaracterising ExOPlanet Satellite (CHEOPS), and the PLAnetary Transits and Oscillations of stars (PLATO) mission, that will greatly benefit from this advanced research to characterize the habitable potential of distant worlds.
Nestled in the habitable zones of stars, exoplanets (like the one in Wolf 1061) are thought to be neither too hot or too cold for liquid water to persist on their surfaces. On Earth, where there's liquid water, there's life, and if there's water on these worlds, there could be life there too. That's the basic logic, but there are many other factors at play that determine whether a planet can indeed support life. So if we can properly characterize exoplanetary atmospheres, we might, some day, be able to detect the chemicals that may reveal information about any "biomarkers" that may be present - chemicals that reveal the presence of biological processes. As Wolf 1061 hosts a small rocky exoplanet (called Wolf 1061c) within its habitable zone, it is one of the closest exoplanetary locations where we could uncover this biological evidence.
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"The Wolf 1061 system is important because it is so close and that gives other opportunities to do follow-up studies to see if it does indeed have life," said Stephen Kane, an astronomer at San Francisco State University and lead author of new research to be published in the Astrophysical Journal.
Working with researchers at Tennessee State University and in Geneva, Switzerland, Kane's team took precise measurements of the Wolf 1061 system to calculate the extent of its habitable zone, stellar activity and planetary orbits. Interestingly, Wolf 1061c has a chaotic orbit that is heavily influenced by the gravity of the other planets in the system, causing it to lurch sometimes closer to the star and at other times further away. It also occupies the inside edge of the star's habitable zone, which poses a quandary for its true habitable potential.
Venus, for example, lies within the inside edge of the sun's habitable zone, yet Venus is anything but "habitable" - despite being approximately Earth-sized. The toxic and thick Venusian atmosphere is the consequence of a runaway greenhouse effect where too much energy has been trapped by the atmosphere, causing it to heat up to lead-boiling temperatures. Though it may have once been a more temperate world, any water that once existed on its surface has been broken down into its component hydrogen and oxygen atoms. The only regions of Venus that are remotely "Earth-like" are high up in Venus' atmosphere - leading to speculative ideas that floating lifeforms may be present, or that humans may one day inhabit Venus in "cloud cities" that float high above the crushing lower atmospheric pressures.
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Now that we've found Wolf 1061c, perhaps it is also an "exo-Venus", though the variability in its orbit may create bursts of global cooling followed by intense warming. "It could cause the frequency of the planet freezing over or heating up to be quite severe," said Kane in a statement.
Like the vast majority of worlds found within stars' habitable zones, Wolf 1061c's Earth-like qualities may be limited to its size and approximate orbital distance from its star - but that doesn't mean it can't host extraterrestrial life, it just means it will likely be very different life to what we are accustomed to on Earth.
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