Can Life's Fingerprint Be Found On Super-Earths?
Image: Kepler-16b is the first exoplanet disc
Exquisite Exoplanetary Art
Sept. 19, 2011 --
They're alien worlds orbiting distant stars far out of reach of detailed imaging by even our most advanced telescopes. And yet, day after day, we see vivid imaginings of these extrasolar planets with the help of the most talented space artists. The definition of an extrasolar planet -- or "exoplanet" -- is simply a planetary body orbiting a star beyond our solar system, and nearly 700 of these extrasolar worlds have been discovered so far (plus hundreds more "candidate" worlds). With the help of NASA's Kepler space telescope, the ESO's High Accuracy Radial velocity Planet Searcher (HARPS), French COROT space telescope and various other advanced exoplanet-hunting observatories, we are getting very good at detecting these worlds, but to glean some of the detail, we depend on artist's interpretations of fuzzy astronomical images and spectral analyses. That's the way it will be until we build a vast telescope that can directly image an exoplanet's atmosphere or physically travel to an alien star system. So, with the flurry of recent exoplanet discoveries, Discovery News has collected a few of the dazzling pieces of art born from one of the most profound searches mankind has ever carried out: the search for alien worlds orbiting other stars; a journey that may ultimately turn up a true "Earth-like" world.
Image: An exoplanet passes in front of (or "t
As an exoplanet passes in front of its star as viewed from Earth, a very slight dip in starlight brightness is detected. Observatories such as NASA's Kepler space telescope use this "transit method" to great effect, constantly detecting new worlds.
Some exoplanets orbit close to their parent stars. Due to their close proximity and generally large size, worlds known as "hot Jupiters" are easier to spot than their smaller, more distant-orbiting cousins.
Image: An artist's impression of Gliese 581d,
The primary thrust of exoplanet hunting is to find small, rocky worlds that orbit within their stars' "habitable zones." The habitable zone, also known as the "Goldilocks zone," is the region surrounding a star that is neither too hot nor too cold. At this sweet spot, liquid water may exist on the exoplanet's surface. Where there's water, there's the potential for life.
Credit: David A. Aguilar (CfA)
Usually, exoplanet hunters look for the slight dimming of a star or a star's "wobble" to detect the presence of an exoplanet. However, in the case of Kepler-19c, its presence has been detected by analyzing its gravitational pull on another exoplanet, Kepler-19b. Kepler-19c is therefore the Phantom Menace of the exoplanet world.
Image: A cool world some distance from its st
The habitable zone seems to be the pinnacle of extraterrestrial living. If you're an alien with similar needs to life on Earth, then you'll need liquid water. If your planet exists outside your star's habitable zone, well, you're in trouble. Either your world will be frozen like a block of ice, or boiling like a kettle. But say if your world had the ability to extend your star's habitable zone? There may be some atmospheric factors that might keep water in a comfy liquid state. Even better, if you like deserts, a dry world could even be oddly beneficial.
Image: A "hot Jupiter" and its two hypothetic
Planets with a global magnetic field, like Earth, have some dazzling interactions with the winds emanating from their stars. The high-energy particles bombard the planet's atmosphere after being channeled by the magnetism. A wonderful auroral lightshow ensues. But say if there's an exoplanet, with a magnetosphere, orbiting really close to its star? Well, stand back! The entire world would become engulfed in a dancing show, 100-1000 times brighter than anything we see on Earth.
Credit: Adrian Mann, <a href="http://www.bisb
"Candidate" exoplanets are often mentioned, especially when talking about detections by the Kepler space telescope. But what does this mean? As a world passes in front of its star, slightly dimming the starlight, this isn't considered a "confirmed" exoplanet detection. To make sure that signal is real, more orbital passes of the exoplanet need to be logged before a bona fide discovery can be announced. Until then, these preliminary detections are called exoplanet candidates.
Image: An exoplanet being destroyed by X-rays
Angry Suns, Naked Planets
Exoplanets come in all sizes and all states of chaos. Some might have wonky orbits, others might be getting naked. Other times, they're simply being ripped apart by X-rays blasted from their parent star. Bummer.
Image: Artist's impression shows HD 85512b, a
Super-Earths get a lot of press. Mainly because "Earth" is mentioned. Sadly, most of these worlds are likely completely different to anything we'd call "Earth." And you can forget calling the vast majority of them "Earth-like." It's simply a size thing -- they're bigger than Earth, yet a lot smaller than Jupiter, hence their name, "super-Earth." Easy.
Credit: Adrian Mann, <a href="http://www.bisb
For now, we have to make do with artist's renditions of exoplanets for us to visualize how they may look in their alien star systems. However, plans are afoot to send an unmanned probe to an interstellar destination. Although these plans may be several decades off, seeing close-up photographs of these truly alien worlds will be well worth the wait.
NASA is preparing the TESS observatory (Transiting Exoplanet Survey Satellite) to follow-up on the successes of the planet-hunting Kepler observatory by identifying nearby exoplanets that pass in front of, or “transit,” their stars. A small sample of these worlds will be singled out for further scrutiny if they lie within the habitable zone of the parent star. The habitable zone is the distance from a star where temperatures on a world may allow liquid water to exist on the planetary surface.
Kepler showed us an incredible diversity among planetary systems, and that small planets like Earth greatly outnumber bloated Jupiter-class worlds. But the Kepler planets are typically over 1,000 light-years away, so understanding the environments of these worlds is technologically out of the question — at least for the foreseeable future.
It is a reasonable prediction that the first transiting candidate planet to look for the chemical signature of life will be a world orbiting a nearby red dwarf star. There are about 90 red dwarfs within just 20 light-years of Earth, but only seven sun-like stars.
A transiting planet will allow for measuring the fraction of starlight passing through its atmosphere as well as recording the difference in light from the system when the planet passes behind its star.
TESS’ survey should at least find a few nearby transiting worlds within reach of doing a chemical inventory with NASA’s planned James Webb Space Telescope. At the very least, Webb would have a shot at providing evidence for an ocean on a planet. This would further narrow down the candidates for more detailed studies.
Kepler’s survey found a number of super-Earths, planets several times Earth’s mass, and therefore too small to be gassy so-called ice giants like Uranus and Neptune. But what might the spectral fingerprint of a nearby super-Earth look like? And could we unequivocally deduce the planet is inhabited to everyone’s satisfaction?
Astrobiologists are now modeling super-Earth atmospheres for planets orbiting in the habitable zone of red dwarf stars. It’s time to begin to try and understand and predict the expected signature of an alien biosphere.
A set of models developed by J. L. Grenfell of the Zentrum fur Astronomie und Astrophysik, Technische Universitat Berlin, and colleagues, start out with the assumption the planet is blanketed with life (unlike Mars where apparently nothing survives on the surface). This would likely require that the target world be several billion years old to allow for the evolution of life to expand, diversify, and significantly modify the planet’s atmosphere.
Life on any planet would use chemical reactions to extract energy, store it, and release certain gases as a byproduct of their metabolism. On Earth the biggest chemical signature of metabolism is oxygen produced by photosynthesis. Other strong signals would come from the presence of ozone and nitrous oxide. Other biotracers, carbon dioxide and methane (already detected on and exoplanets by the Hubble Space Telescope) can also be produced by non-biological processes.
The researchers find that ozone levels can vary widely given other conditions on the super-Earths. In particular ultraviolet radiation from a red dwarf star is anemic. Without UV radiation to break apart certain gasses a red dwarf planet might have a higher concentration of biosignature gasses, as well as a smoggy sky.
The model also looks at surface gravities that could be as high as three times that of Earth’s. This might inhibit the movement of biogases from the planet’s surface into the higher stratosphere. What’s missing in the models are such critical but unknown variables are whether the planet has a shielding magnetic field, or plate tectonic for recycling atmospheric gases like carbon dioxide that otherwise might build up to trigger a runaway greenhouse effect.
The bottom line is that super-Earth atmospheres will be complicated and messy and will probably not look like anything that unequivocally settles the question of habitability. This will spur the far-future goal of building interstellar probes to visit the nearest habitable planet candidates.
“A sobering thought usually left unacknowledged is that when we finally discover biosignature gasses in may not be the triumphant 100 percent certainty,” cautions MIT astronomers Sarah Seager in a recent essay in Science magazine. She says that astrobiologists will be left with an “assigned probability” depending on the level at which the likelihood of a false positive can be estimated. “Planet habitability is planet specific,” she concludes.
Image Credits: NASA, D. Agular/CfA