January 24, 2014
Study Suggests Existence Of ‘Superhabitable’ Exoplanets
[ Watch the Video: Are Some Exoplanets More Super Than Earth? ]
Brett Smith for redOrbit.com - Your Universe Online
The stellar habitable zone (SHZ), or Goldilocks zone, is considered the range of ideal orbital distances from a star for creating the conditions necessary for life. A new paper published recently in the journal Astrobiology argues that some planetary bodies outside this zone may hold environments even more habitable for life than those found here on Earth. The authors suggest that orbital and rotational energy released as heat in the crust of planetary bodies, also known as tidal heating, can create conditions in which life could emerge on a planet or moon once considered uninhabitable.
"A great place for hydrothermal microorganisms and a volcanic eruption in the weather forecast every morning and evening, a tidally heated planet would be unpleasant though spectacular to visit,” said Norman Sleep, an editor for Astrobiology and earth sciences professor at Stanford University.
In the paper, René Heller from McMaster University in Canada and John Armstrong from Weber State University in Utah posited that every habitable world does not need to be situated in the SHZ and worlds in the SHZ are not necessarily habitable.
“Scientists have developed a language that neglects the possible existence of worlds that offer more benign environments to life than Earth does,” the authors wrote.
The physicists said uninhabitable planets can typically be classified as being too frigid for life, a “snowball” state, or too hot for life – possibly caused by a thick layer of atmospheric insulation, or a runaway greenhouse state. They noted that tidal heating can be a heat source that causes a planet, like Venus, to go into a runaway greenhouse state and become uninhabitable. Conversely, tidal heating could make a world orbiting outside the HZ inhabitable by preventing it from entering a snowball state.
“Tidal heating could partly compensate for the reduced stellar illumination beyond the stellar HZ and potentially maintain liquid water reservoirs,” they wrote.
The scientists went on to say that Earth could be just one of many types of planets capable of hosting life and may possibly be even less hospitable than some.
“From a potpourri of habitable worlds that may exist, Earth might well turn out as one that is marginally habitable, eventually bizarre from a biocentric standpoint,” they wrote.
The authors added that assuming Earth is the ideal planet for life to emerge “could mislead research for extrasolar habitable planets because planets could be non-Earth-like but yet offer more suitable conditions for the emergence and evolution of life than Earth did or does; that is, they could be superhabitable."
Assuming liquid water is a prerequisite for life, the researchers said a superhabitable planet would need a terrestrial surface area larger than Earth’s that permits liquid water. They noted that Earth is the largest terrestrial planet in the Solar System.
According to the authors, other characteristics of superhabitable planets include: early planetary bombardment, planetary spin, stellar UV irradiation an atmosphere and biological diversification.
In their conclusion, the authors noted that the K1V star Alpha Centauri B, which is a member of the closest stellar system to the Sun, is thought to host an Earth-mass planet in a 3.2-day orbit. The said this star is “an ideal target for searches of planets in the HZ and, ultimately, for superhabitable worlds.”