Oceans May Be Common on Rocky Alien Planets
This artist's conception shows the inner four planets of the Gliese 581 system and their host star. The large planet in the foreground is Gliese 581g, which is in the middle of the star's habitable zone and is only two to three times as massive.
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.
Every rocky planet likely develops a liquid-water ocean shortly after forming, suggesting that potentially habitable alien worlds may be common throughout the universe, a prominent scientist says.
The building blocks of rocky planets contain more than enough water to seed oceans, and computer models and Earth's own history suggest such seas should slosh around soon after these worlds' surfaces have cooled down and solidified, said Lindy Elkins-Tanton of the Carnegie Institution for Science in Washington, D.C.
"Habitability is going to be much more common than we had previously thought," Elkins-Tanton said today (March 18) during a talk at the 44th Lunar and Planetary Science Conference in The Woodlands, Texas.
Making an early ocean
Analysis of ancient Earth rocks shows that our own planet hosted an ocean of liquid water at least 4.4 billion years ago, Elkins-Tanton said -- just 160 million years or so after our solar system's birth. (9 Exoplanets That Could Host Alien Life)
This water came primarily from the planetesimals that glommed together to form Earth long ago rather than from comet impacts, as some researchers had previously believed, she added.
While comet-delivered water probably made a contribution later on, "it's not required," Elkins-Tanton said, citing studies that model planetary building blocks and how they come together. "You can make a water ocean without it."
For example, even if the pieces that built Earth contained just 0.01 percent water by weight -- an extremely conservative estimate -- our planet still would have harbored an early global ocean hundreds of meters deep, she said.
Such primitive oceans form in a multistep process, Elkins-Tanton explained. Water first boils out of the molten rock covering a newborn terrestrial planet heated up by accretionary impacts, creating a steamy atmosphere. This atmosphere then collapses as the planet cools, returning the water to the surface and generating an ocean.
"The ramifications of this are that, in any exoplanet system anywhere in our universe, if it's made of rocky materials with similar water contents to ours, every rocky planet would be expected to start with a water ocean," Elkins-Tanton said.
Further, models developed by Elkins-Tanton and others "all indicate that this cooling and collapse process happens on the order of 10 million years or less," she added.
That's an exciting prospect for astrobiologists, as life on Earth is found nearly anywhere liquid water exists.
Holding on to the water
Of course, forming an ocean and holding onto it are two different matters. After all, Earth's solar system hosts rocky planets -- Mercury, Venus and Mars -- whose surface oceans have long since disappeared, if they ever existed at all.
Indeed, how some rocky worlds manage to retain their water is an area ripe for future research, Elkins-Tanton said, specifically citing the case of Venus, Earth's hellishly hot "sister planet" that veered down a very different road after its formation.
It may be tempting to ascribe the apparent dessication of rocky worlds like Venus to the giant impacts that pummeled them in our solar system's early days. But Earth held onto much of its water despite a massive collision with a Mars-size body (which is thought to have led to the formation of the moon), and data from NASA's Messenger spacecraft show that Mercury still harbors many volatile compounds, Elkins-Tanton said.
"Now if there would be a poster child for the body that should be depleted by giant impacts, that would be Mercury," Elkins-Tanton said. "Giant impacts do not dry bodies."
More from Space.com:
The Strangest Alien Planets (Gallery)
Planets Large and Small Populate Our Galaxy (Infographic)
5 Bold Claims of Alien Life
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