New Thoughts on Habitable Planets
Astrobiologists are trying to standardize the requirements for habitable planets. Do they get them all?
The search for life beyond the earth can’t waste time looking at every star. How should astrobiologists find the needle in the haystack? One promising method is to focus on the right kind of stars, and among those, the ones that have planets in the “habitable zone.” But what is meant by “habitable”? Historically, the habitable zone was defined by an inner and outer orbital radius where water could exist on the surface of a planet. That might be too simplistic, some think, but it might be too restrictive also: what about distant planets with an ocean under an icy crust?
In order to formalize the concept of habitability, astronomers at the University of Washington have devised a “habitability index” for planets, Astrobiology Magazine says (see also Science Daily). This new method of scoring planets will help when the James Webb Space Telescope, with advanced detection instruments, launches in 2018. It will also help filter through the hundreds of candidate planets detected by the Kepler Telescope. The work was funded by NASA’s Astrobiology Institute.
Because the old “habitable zone” definition was admittedly too simplistic, “The new index is more nuanced, producing a continuum of values that astronomers can punch into a Virtual Planetary Laboratory Web form to arrive at the single-number habitability index, representing the probability that a planet can maintain liquid water at its surface.” Here are three of the additional factors considered in the new index:
- Estimates of a planet’s rockiness, with rocky planets being the more Earth-like.
- “A phenomenon called ‘eccentricity-albedo degeneracy,’ which comments on a sort of balancing act between the planet’s albedo — the energy reflected back to space from its surface — and the circularity of its orbit, which affects how much energy it receives from its host star.”
- Closeness to the inner or outer edges of the habitable zone, which will affect other factors that must compensate for the heat or cold.
The team knows that the new habitability index parameters will require periodic updating and rethinking. Victoria Meadows, a UW astronomy professor, says, “This innovative step allows us to move beyond the two-dimensional habitable zone concept to generate a flexible framework for prioritization that can include multiple observable characteristics and factors that affect planetary habitability.” But can a single number be meaningful, if other requirements are missed?
Don’t Forget the Magnet
Most likely the list of factors will grow, not shrink. Science Daily pointed out another factor being considered by the UW astronomers (in work also funded by NASA Astrobiology): magnetic fields. “Earth-like planets around small stars likely have protective magnetic fields,” the headline says. That’s because a watery planet could be sterilized by high-energy radiation from its star and from space.
That article mentions another problem: tidal locking. Dwarf stars, being the most abundant in the universe, have a problem: they tend to lock one face of their planets to the star since the habitable zone of cooler stars is closer in. Peter Driscoll wondered if that not only reduces the temperate regions of the planet, but “are these planets going to be roasted by gravitational tides?” In order “to produce a more realistic picture of what is happening inside these planets,” he and his assistant ran models that suggest the problem is not as serious as thought. But the smaller the star, the more extreme the tidal heating becomes. “These preliminary results are promising, but we still don’t know how they would change for a planet like Venus, where slow planetary cooling is already hindering magnetic field generation,” Driscoll said.
Don’t Play Pinball
Astronomers gasped ‘Hot Jupiter!’ when the first gas giants around other stars were found orbiting closer than Mercury to the sun – a discovery that sent modelers scrambling to explain how they got there. This led to realizations that if they formed far out, like our gas giants, they must have migrated inward. Slow processes of core accretion had to give way to this new puzzle. Science Daily reports that French astronomers deduce that ‘Hot Jupiter’ exoplanets “may have formed very rapidly” – why? – because a big one has been inferred to exist around a young star. This means it probably formed early and rapidly, and yet has not had time to crash into the star. The article doesn’t go into this, but a big planet 1.5 times the mass of Jupiter (as inferred) would wreak havoc with any rocky planets in the star’s habitable zone. Add that factor to the habitability index.
Does a Jupiter help? The jury is out on this, Fraser Cain says in PhysOrg. Some astronomers like Kevin Grazier, formerly at JPL, think that Jupiter is just as likely to aim comets at the Earth as fling them away. More modeling will be required to see if Jupiter is our friend or enemy. Till then, it will be hard to input this number in the habitability index. For life on a rocky planet, one comet impact can ruin your whole day.
Don’t Forget the Veggies
Patrick M. Shih considers the history of photosynthesis on Earth in Current Biology. Getting a planet to utilize energy is part astronomy, part geology, and part biology—and all are interrelated.
Just as geological determinants have shaped the course of biology, life has undoubtedly changed the geological course of Earth. One key biological innovation that has contributed to these geochemical cycles is the ability to use light energy to drive the metabolic process of generating chemical energy. Photosynthesis has been instrumental in the success of life on Earth, and life has had to adapt and evolve in the face of dramatic geological perturbations over billions of years.
Instrumental in the origin of photosynthesis is a stable star producing the right kind of energy. “The Sun sustains the vast majority of life on Earth,” he realizes. But not just any sun will do. In a recent video, Privileged Species, Michael Denton pointed to numerous coincidences between our sun’s energy spectrum, Earth’s atmosphere, mineral availability in the crust, elemental abundances (particularly oxygen), and atomic physics that make photosynthesis possible. So if aliens on other planets want to be vegetarian, they will need to include these factors in their habitability index.
On the flip side, would the detection of oxygen in an exoplanet’s atmosphere signal the presence of life? Not necessarily, Space.com says. “Although scientists have long considered oxygen a sign that life exists on an alien planet, new research suggests the element could be produced without it.”
New ideas about core formation were announced by PhysOrg. “There is more oxygen in the core of Earth than originally thought,” and less silicon, says Rick Ryerson in a paper in PNAS. His model depends heavily on theories of planetary accretion. “This new model is at odds with the current belief that core formation occurred under reduction conditions,” Ryerson said. “Instead we found that Earth’s magma ocean started out oxidized and has become reduced through time by oxygen incorporation into the core.” Effects of this on origin-of-life theories is not stated, but oxygen is the enemy of organic molecules produced in Miller-type experiments. Once cells boot up, though, they cannot evolve past the methanogen stage without oxygen. They probably had to hide out far below the surface, too. That’s because oxygen is important for the ozone layer that protects the Earth’s surface from UV radiation. Add those factors to the habitability index.
The Missing Ingredient
All the above assumes that life can, or perhaps will, form on a habitable planet. What they’re missing, a new film from the Discovery Institute says, is information. The Information Enigma focuses on the centrality of information – digitally encoded, programmed information – to life. Doug Axe’s calculations show that the probability of getting functional information by chance is unfathomably tiny, like trying to solve a combination lock with 150 positions. Stephen Meyer asks what is a cause in operation today that produces digital information? There’s only one we know of: it’s intelligence, he says. This means that a naturalistic origin of life is scientifically irrational, despite Current Biology‘s optimistic take on the old RNA World scenario. If life on other planets exists, Meyer concludes, we can infer it was designed. But life needs all these habitability requirements, too. This implies that the design of life extends to all the planetary habitability factors that sustain it.
Hope against hope; that’s astrobiology. Their optimism and realism pull in opposite directions. Watch the new Discovery Institute video The Information Enigma, and also Privileged Species. These are great new video resources that you can share on social media to help our graphic-oriented culture. We also remind you of The Privileged Planet, where you can see Kevin Grazier talk about another factor: the importance of having a moon like ours. There’s just too much beauty, elegance and perfection in Earth to think chance got it right. There’s only one necessary and sufficient cause that produces beauty, elegance and perfection: that’s intelligence. Trust the cause we know of, not blind chance.
NASA, here’s a suggestion. Instead of teasing and tantalizing the public about life on other planets (which has no scientific support and massive problems), why not focus on what exoplanet research is telling us about habitability on Earth? Teach people all about the privileges we have here compared to all the other bodies in the solar system and around other stars. People will like that, and you can continue spending money on research. It will support environmentalism, too, the more we realize how special Earth is. This is a positive proposal to bring your realism into alignment with your optimism.