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.
Through the combined power of two NASA space telescopes, the clouds hanging high in an alien atmosphere have been mapped for the first time. But don’t go having romantic dreams of white puffy cumuli drifting over the landscape, these clouds are likely way beyond anything we could possibly imagine in our temperate biosphere.
Until recently, extrasolar planets — or exoplanets — could only be detected indirectly. But with the help of increasingly sophisticated optics, direct observations of these alien worlds have been possible. However, the “Holy Grail” of exoplanet studies will only be attained when we can directly probe the characteristics of the atmospheres of the smaller planetary bodies. Driving this exoplanetary fascination is the hunt for alien life and habitable worlds, but in the case of Kepler-7b — which has been studied by NASA’s Kepler and Spitzer space telescopes — it’s just the fact that we can see cloud structure in its atmosphere that has left astronomers buzzing.
“By observing this planet with Spitzer and Kepler for more than three years, we were able to produce a very low-resolution ‘map’ of this giant, gaseous planet,” said Brice-Olivier Demory, of Massachusetts Institute of Technology in Cambridge. “We wouldn’t expect to see oceans or continents on this type of world, but we detected a clear, reflective signature that we interpreted as clouds.”
Data from NASA’s Kepler space telescope has confirmed the discoveries of over 150 exoplanets (with hundreds more candidate exoplanets) of which Kepler-7b was one of the first of the mission’s confirmed worlds. The exoplanet orbits very close to its host star and is a puffed-up “hot-Jupiter” — it is about half the mass of Jupiter and yet 1.5 times the gas giant’s physical size.
Kepler has recently ceased its exoplanet-hunting operations due to a failed reaction wheel, but during its four years of science operations, the orbiting telescope watched planets pass in front of their host stars, creating small dips in starlight brightness as they did so. While watching for these “transits,” more information could be gleaned from the reflected starlight off Kepler-7b’s atmosphere. The moon-like phases observed during the world’s orbit around its star revealed bright structure in the exoplanet’s western atmosphere.
During follow-up infrared studies by Spitzer, the temperature of the structure could be gauged. Spitzer deduced that Kepler-7b’s atmosphere is between 1,100 and 1,300 Kelvin (1,500 and 1,800 degrees Fahrenheit), much cooler than would be expected for an atmosphere of a planet that orbits so close to its star.
“Kepler-7b reflects much more light than most giant planets we’ve found, which we attribute to clouds in the upper atmosphere,” said Thomas Barclay, Kepler scientist at NASA’s Ames Research Center in Moffett Field, Calif. “Unlike those on Earth, the cloud patterns on this planet do not seem to change much over time — it has a remarkably stable climate.”
Stable it may be, but the atmospheric conditions on Kepler-7b are hellish — hot with (likely) powerful global winds driven by a star only 0.06 AU (6 percent of the Earth-sun distance) away.
This study is an exciting glimpse into the possible future of observing and characterizing exoplanets; particularly smaller, Earth-sized worlds that possess temperate environments possibly habitable to life as we know it. Detecting cloud and other atmospheric features of small alien worlds will help us understand the frequency of true “Eath-like” planets.
“With Spitzer and Kepler together, we have a multi-wavelength tool for getting a good look at planets that are trillions of miles away,” said Paul Hertz, director of NASA’s Astrophysics Division in Washington. “We’re at a point now in exoplanet science where we are moving beyond just detecting exoplanets, and into the exciting science of understanding them.”
The Kepler-7b study has been accepted for publication in the Astrophysical Journal Letters.
Image: Kepler-7b, which is 1.5 times the radius of Jupiter, is the first exoplanet to have its clouds mapped. The cloud map was produced using data from NASA’s Kepler and Spitzer space telescopes. Image credit: NASA/JPL-Caltech/MIT