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.
There are few celestial objects that inspire such vivid imaginings than carbon-rich planets. Carbon, of course, is the key ingredient in diamonds. So if you follow the logic, carbon-rich alien worlds may evolve into exotic planets with spires of diamonds jutting out of the surface, or layers of diamond-rich gravel, glistening in the starlight.
However, theoretical models of these carbon-rich worlds conclude that ‘diamond planets’ probably aren’t life’s best friend.
In research presented at the American Astronomical Society Division of Planetary Sciences meeting in Denver, Colo., on Oct. 7, Torrence Johnson of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., highlighted the challenges life (as we know it) would face in carbon-rich planetary systems.
“The building blocks that went into making our oceans are the icy asteroids and comets,” he said. “If we keep track of these building blocks, we find that planets around carbon-rich stars come up dry.”
The problem, according to Johnson and his colleagues, is that the extra carbon found in carbon-rich proto-planetary systems would lock onto oxygen, preventing the oxygen from chemically bonding with hydrogen. Of course, the key substance for life on Earth is water, a.k.a. H2O — two hydrogen atoms combined with one oxygen atom. It seems that, in this scenario, life would be snuffed-out before it could even begin.
“It’s ironic that if carbon, the main element of life, becomes too abundant, it will steal away the oxygen that would have made water, the solvent essential to life as we know it,” said research collaborator Jonathan Lunine of Cornell University, Ithaca, New York.
We take water for granted on our planet and throughout the solar system, but “all rocky planets aren’t created equal,” said Lunine.
Our sun was formed by the gravitational collapse of a rich cloud of gas and dust that originated from older generations of stars that exploded as supernova or fizzled out of existence billions of years ago. It just so happens that the carbon-to-oxygen ratio from this stellar gas soup was just right for our sun to form silicate-rich rocky worlds like Earth (as opposed to carbon-rich worlds) with the perfect amount of water ice accumulating beyond the “snow line” of the solar system. The snow line is the distance at which a given substance will freeze, coalesce and accumulate. Asteroids and comets collected their abundance of water in this region — thought to be the source of the liquid water oceans on Earth.
However, in models where the carbon-to-oxygen ratio was varied to favor the formation of carbon-rich worlds, no water formed. In this case, “there’s no snow beyond the snow line,” said Johnson.
This creates an interesting complexity to the search for “habitable” exoplanets. Although a candidate “Earth 2.0″ may be the same size as our planet and orbit within its star’s “habitable zone” — the region surrounding a star where it’s not too hot and not too cold for liquid water to exist on a planetary surface — if it is carbon-rich, it could be the antithesis of “habitable.”
“So-called diamond planets the size of Earth, if they exist, will look totally alien to us: lifeless, ocean-less desert worlds,” Lunine concluded.
Publication: “Planetesimal Compositions in Exoplanet Systems,” Torrence V. Johnson, Olivier Mousis, Jonathan I. Lunine, Nikku Madhusudhan, 2013. arXiv:1208.3289v1 [astro-ph.EP]
Image: This artist’s concept illustrates the fate of two different planets: the one on the left is similar to Earth, made up largely of silicate-based rocks with oceans coating its surface. The one on the right is rich in carbon — and dry. Credit: NASA/JPL-Caltech