Artist's impression of extrasolar planet HD 189733b with its parent star peeking above its top edge. Astronomers used the Hubble Space Telescope to detect methane and water vapor in the Jupiter-size planet's atmosphere in 2008. They made the finding by studying how light from the host star filters through the planet's atmosphere.
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.
As astronomical instrumentation becomes more sophisticated, we are rapidly approaching a crossroads in the search for extraterrestrial life, according to a leading planetary scientist. It’s also “inevitable” that alien life exists in the universe given the preponderance of extrasolar planets that are being discovered — it’s up to us to seek out the extraterrestrial biosignatures.
These conclusions are outlined by Sara Seager, Professor of Planetary Science and Physics at the Massachusetts Institute of Technology (MIT), in a paper published in the journal Proceedings of the National Academy of Sciences on Aug. 4.
“In the coming decade or two, we will have a lucky handful of potentially habitable exoplanets with atmospheres that can be observed in detail with the next generation of sophisticated space telescopes,” writes Seager, pointing out that NASA’s James Webb Space Telescope (JWST) and a planned direct-imaging space telescope will be able to seek out biosignatures (i.e. chemicals created by extraterrestrial biology) in the atmospheres of nearby exoplanets. The JWST is set for launch in 2018.
“Life can be inferred by the presence of atmospheric biosignature gases — gases produced by life that can accumulate to detectable levels in an exoplanet atmosphere,” she writes.
To date, a handful of exoplanetary atmospheres have been studied through the analysis of their host star’s light passing through their atmospheres. As an alien world orbits its star, from our perspective, it may block some of the starlight from view and be registered as a “transit.” The transit method is used by NASA’s Kepler space telescope and has so far confirmed the detection of hundreds of exoplanets. But this method can also help us analyze the chemicals contained in exoplanetary atmospheres.
During a transit, if that exoplanet has an atmosphere, some of the starlight is filtered through its atmosphere. Some wavelengths of that light are absorbed by specific chemicals, leaving a spectroscopic ‘fingerprint’ in the starlight we detect. Although only the largest class of exoplanets have so far had their atmospheres analyzed in this way (gas giants with tight orbits around their stars known as “hot-Jupiters”), Seager argues that with the advent of advanced space telescopes, the composition of smaller worlds’ atmospheres could also studied. Habitable “super-Earths” fall into this category.
Once this happens, we can begin to observe small rocky worlds, potentially detecting spectroscopic signatures of chemicals associated with life.
Although the next generation of space telescopes may be able to detect biosignatures in nearby exoplanets, Seager urges caution.
“(M)any different gases are produced by life, but the anticipated diversity of exoplanet atmosphere composition and host star environments may yield different detectable biosignature gases than the terrestrial examples. Even with excellent data, false positives will drive a permanent ambiguity in many cases,” she adds.
Molecules such as methane can be generated through biological (methanogenic) and geological (volcanic) processes, so the detection of methane in an exoplanetary atmosphere may not indicate life. To find out what is generating that gas, astronomers will need to study the atmosphere in its entirety to avoid jumping to conclusions about that world’s biological potential. The identification of these “false positives,” using advanced instrumentation, will be critical when seeking out genuine biosignatures.
The advances in space-based observatories are tantalizing and, with the launch of JWST and other advanced direct imaging telescopes (such as the “star shade” concept), we could start studying small habitable worlds with atmospheres and teasing hints as to any biosignatures within the next couple of decades.
But to fully investigate this exciting class of exoplanet, “we require the ability to directly image exoplanets orbiting 1,000 or more of the nearest sun-like stars.” Such an endeavor would require a huge space-borne observatory — an optical telescope with a diameter exceeding 10 meters. Considering the Hubble Space Telescope is only 2.4 meters in diameter, the exoplanetary atmosphere telescopes of the future will require some huge innovative leaps before they become a reality.
One thing seems certain, however. The longer we gaze into the stars, the more certain we become about the possibility for life beyond Earth.
“Our own Galaxy has 100 billion stars, and our Universe has upwards of 100 billion galaxies — making the chance for life elsewhere seem inevitable based on sheer probability,” writes Seager. “We can say with certainty that, for the first time in human history, we are finally on the verge of being able to search for signs of life beyond our solar system around the nearest hundreds of stars.”