David A. Aguilar/CfA
A habitable exoplanet orbits a white dwarf star -- could systems like these reveal the chemistry of exoplanetary atmospheres?
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.
With the list of planets beyond the solar system growing almost daily, scientists are eager to learn if any of the distant worlds actually are hosting life.
But even with a new generation of telescopes, scanning Earth-sized planets for the chemical fingerprints of life will be a time-consuming and tedious process.
First, scientists have to find planets that pass in front of their parent stars, relative to Earth’s line of sight, work that already is under way with NASA’s Kepler space telescope. Of particular interest are planets positioned the proper distance from their host stars so that, like Earth, they could have the right temperatures for liquid water on their surface. Water is believed to be necessary for life.
The trickiest step, however, will be to capture and analyze the tiny bit of starlight that passes through a transiting planet’s atmosphere for signs of oxygen and other potential biomarkers.
Two astronomers think they can better the odds by homing in on a particular type of star called white dwarfs, which are the leftover remains of dead stars that have shrunken down to about the size of Earth.
It is precisely their diminutive size that make white dwarfs an attractive target in the hunt for life beyond Earth, says Harvard University astronomer Avi Loeb.
During a transit, an Earth-sized planet in a white dwarf’s habitable zone would block out a sizable portion of its parent star’s light, if not completely occult the star. That geometry allows for a much quicker and easier scan of a planet’s atmosphere.
“If there is an atmosphere around the planet, this is a great opportunity to actually learn about the composition,” Loeb told Discovery News.
The idea of a dead star hosting planets is not as far-fetched as it may sound. Astronomers have found that at least 20 percent of white dwarf stars are littered with heavy elements and debris rings that are believed to be the shredded remains of planets.
Whether any planets could exist in a white dwarf’s habitable zone, which is about 100 times closer to the star than Earth is to the sun, has yet to be determined.
“We expect to find planets in stable orbits around white dwarfs. The tricky part is to find them in the habitable zone. Because the stars are so much smaller, the habitable zone is going to be very close in,” said astronomer Carolyn Brinkworth with the California Institute of Technology.
One way a planet might find itself in a white dwarf’s habitable zone is if it were gravitationally kicked inward by a larger planet farther away.
“Obviously, you don’t rule anything out, but I think it would take a very, very well-placed kick and luck to get something into a habitable zone. I think your chances of that are fairly slim,” Brinkworth told Discovery News.
Nevertheless, Loeb argues it’s worth a look.
“We know from the history of astronomy that whenever people tried to guess what the sky is like, they were very often wrong,” Loeb said.
Much of the groundwork for scanning suitable white dwarf stars can be done with small telescopes that exist today. Then, after NASA’s James Webb Space Telescope is in orbit, follow-up observations of prime candidates can be done in just five hours, computer models show.
“It’s practical to do it and it doesn’t require huge amounts of funds. You can use existing facilities and existing surveys that people will do anyway. You just have to have this goal in mind and not be distracted by prejudice,” Loeb said.
“Some people would say ‘Oh, but these are environments are so different than what we are used to. We want an Earth-like planet around a sun-like star.’ The problem is it’s impossible with existing facilities -- and with those that will be built in the next decade -- to find evidence of oxygen, for example, in those atmospheres.
“We outline a practical approach with which you can search for those biomarkers,” Loeb said. “This is the best target to go after with a reasonable chance of finding something in the next decade.”
The research appears in the Monthly Notices of the Royal Astronomical Society.