NASA’s prolific Kepler space observatory never ceases to amaze planet hunters. Last week’s announcement of two “super-Earth” type planets sharing the habitable zone around the red dwarf star Kepler-62 further ratcheted up our optimism that life-bearing planets are all over the galaxy.
Kepler scientists explained that the only data the stellar transits (when the planets pass in front of their stars) have to tell us is the planets’ orbits and diameters. Still it’s hard for experts to resist speculating that these planets “may have polar caps,” or may be “water worlds,” or, “we may have found the first ocean planet,” The scientists finally did acknowledge in the April 18th NASA press conference that they were doing “a lot of arm waving.”
The same would be true for my imaginary alien astronomer Zork, on a planet 1,000 light years away. Zork reports to colleagues that photometric observations of the yellow dwarf star Sol (a.k.a. our sun) reveals three planets inside or on the edge of Sol’s habitable zone: Venus, Earth and Mars. He considers this a great SETI target because you get three inhabitable worlds for the price of one observation!
Because we live here we know the reality is that only Earth can support advanced life. Venus and Mars could have started out habitable, with oceans on their surfaces, but both took sharp turns in planetary evolution. Venus’ oceans evaporated away and Mars’ ocean froze over.
When pondering the inhabitability of planets for advanced life, we need to consider planetary evolution and how it changes the weather and surface environments of the planets. The first question is: how much water was available to these planets when they formed? Our terrestrial planets may have been irrigated by asteroids slingshot toward the inner solar system by Jupiter’s gravity.
But what happens in systems without Jupiters? A 2010 study by Rebecca Martin of the University of Colorado reported that only a tiny fraction of planetary systems observed to date seem to have giant planets in the right location to produce an asteroid belt of the appropriate size to influence inner planets. If it’s more massive the planets are rototilled by frequent asteroid hits — and this might not be good for biological evolution.
Another study by Martin found that our terrestrial planets formed from rocky debris in a dry, hotter region, far inside of the solar system’s so-called “snow line.” The snow line currently lies in the middle of the asteroid belt. Beyond this point the sun’s light is too weak to melt the icy debris left over from the protoplanetary disk. Conditions within disks will vary from star to star and so the solar system’s comparative dryness cannot give us insights into Kepler-62.
Compounding the astrobiology question is the fact that Earth’s own evolution shows how a planet even in the habitable zone can dramatically change over time. Four billion years ago Earth had a thick carbon dioxide atmosphere and green oceans full of iron. At 700 million years ago Earth was entombed in ice because of runaway glaciation triggered by changes in ocean currents caused by the appearance of a super-continent. Earth has only had a rich surface biosphere for the past 500 million years.
Therefore, planetary evolution keeps shuffling the playing deck. The inner planet, Kepler-62e, may really be a super-Venus rather than super-Earth. It would be a hellish place with a chocking carbon dioxide atmosphere and continuous volcanism. One the other hand could “Venus be just a fluke?” speculated one Kepler scientist.
Kepler-62 is a 7 billion year-old system, it’s got the jump on us by 2.5 billion years. So any Earth-like planet may have evolved into unknown territory for astrogeologists.
What’s tantalizing is that anyone living in a 7 billion-year-old system should be a lot smarter than us! If Kepler-62f and 62e were inhabited, space-faring civilizations might visit each other. My guess is that the atmospheres could be so different that visiting species might have a hard time without wearing spacesuits, and this would preclude any attempt at a preemptive interplanetary invasion. Inhabitants of either planet would need awfully powerful rockets to escape stronger gravitational fields. They might be restricted to twittering through interstellar radio transmissions, or exchanging small payloads.
Likewise, the only way to determine habitability would be through a radio or laser beacon sent earthward by Kepler 62’s inhabitants, as Discovery News’ Ian O’Neill recently wrote. Kepler-62 is so far away there are no telescopes envisioned — or affordable — that would be capable of observations that could be done to assess the planets’ physical characteristics.
Our best bet is NASA’s recently announced Transiting Exoplanet Survey Satellite (TESS). Call it Kepler-2. The TESS is expected to identify approximately 1,000 exoplanets around nearby stars.
Planned for a 2017 launch, TESS will find targets for NASA’s James Webb Space Telescope (JWST) that will study planetary atmospheres in infrared light. Webb has the capability of offering the first observational evidence for a true extraterrestrial ocean.
The JWST will launch only a year after TESS, and so the two observatories will work together in seeking out Earth-clones where habitability can at last be assessed. But for now, it’s all just educated “arm-waving.”
Image credit: NASA