An exoplanet orbiting a red dwarf star may appear to have signatures for life, when in fact those signatures are being generated by non-biological processes.
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.
In the quest for life beyond Earth, a planet with oxygen in its atmosphere is considered the Holy Grail. But appearances can be deceiving.
So says a team of scientists studying a particular type of star known as an M dwarf, or red dwarf. These long-lived stars, which are smaller than the sun, account for about 75 percent of the stars in our galaxy.
Astronomers found that M dwarf stars have relatively high levels of far-ultraviolet radiation -- 1,000 times more than the sun. These emissions could trigger chemical reactions in an orbiting planet's atmosphere that create oxygen and ozone.
"This could be taken as a false positive (for life)," said astronomer Feng Tian, with the Center for Earth System Science at Tsinghua University in Beijing, China.
"If you observe these planets’ atmospheres, you’ll see oxygen and you may think ‘Oh the oxygen could come from life, like plants on our own planet,’ but actually that is not the case," Tian told reporters during a webcast press conference at the American Astronomical Society meeting in Denver this week.
The discovery has implications for a new generation of planet-hunting observatories, such as NASA’s planned Transiting Exoplanet Survey Satellite, or TESS, a follow-on to the now-defunct Kepler space telescope.
Like Kepler, TESS will look for minute changes in the amount of light coming from target stars, which could be caused by planets passing by relative to the telescope’s line of sight. For small host stars, like M dwarfs, a transiting planet would block proportionally more of its light, making the detection easier.
"M dwarfs have been considered the fast track for the search of extraterrestrial life," Tian said.
"In order to understand whether there is life on these planets, we need to look at the photochemistry," he added
Tian and colleagues used Hubble Space Telescope data to look at the ultraviolet emissions from four M dwarf stars, including one that has three planets located in its so-called “habitable zone” where temperatures are suitable for liquid surface water.
In related research, another team of astronomers pointed out that carbon-rich systems may be deplete of water-bearing bodies, dimming prospects for life.
"Planets in the habitable zones of these systems might have lots of other interesting things. They might have carbon. They might have organics. But they might lack a key ingredient -- water,” said Torrence Johnson, a senior research scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.