'Rock Snow' May Fall From Exoplanet Skies
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.
Eight years ago, astronomers announced the discovery of 51 Pegasi b (51 Peg b), the first planet found circling a sun-like star beyond our solar system.
The planet is about the size of Jupiter, but it orbits far closer to its parent star than Mercury orbits the sun. A year on 51 Peg b lasts just four Earth days.
That got Brian Jackson, a researcher at the Carnegie Institution for Science in Washington DC, and colleagues curious if there were planets located even closer to their host stars. So they began sifting through nearly three years of data collected by NASA’s Kepler space telescope.
The observatory, now sidelined by a positioning system failure, looked for slight dips in the amount of light coming from target stars, potential telltale signs of planets passing by, or transiting, relative to the telescope’s line of sight.
Kepler was designed to seek out Earth-like worlds fortuitously positioned from their parent stars for liquid surface water, the so-called “Goldlilocks” zone believed to be favorable for life.
But staring at 150,000 stars for four years turned up thousands of other potential planets, including four candidate planets with orbits 20 times closer than Mercury circles the sun.
Surface temperatures on these planets would soar past 3,000 degrees Fahrenheit -- hot enough to melt rock.
One suspected planet, referred to as KOI-1843, circles its host star in 4.2 hours -- an orbit that brings it 40 times closer to its star than Mercury orbits the sun -- with a surface temperatures of about 4,200 degrees Fahrenheit.
Conditions on these planets would be extreme. Surfaces on their day-sides are likely molten rock, setting up the bizarre prospect of rocky vapor atmospheres that transform into “rock snow” falling on their relatively cooler night-sides.
“That could be very interesting,” Jackson told reporters during a webcast press conference at the American Astronomical Society meeting in Denver this week.
“These molten rock lakes could shed a rocky vapor atmosphere that could go screaming around to the night side of the planet and then be deposited as sort of a rock snow as the rock vapor cools,” Jackson said.
While life on these worlds is highly doubtful, finding extrasolar planets that skim their parent stars is helping scientists figure out how planets form and evolve.
“They didn’t form there, almost definitely,” Jackson told Discovery News.
For starters, temperatures in the original gas disks from which the planets formed would have been too high at the planets' current locations for them to form, Jackson pointed out.
Also, the parent stars were much bigger in their youth, putting the planets inside the stars, he added.
“They must have formed much farther out and then were brought in by some processes or combination of processes. We don’t really know how they got so close,” Jackson said.
The research has been submitted to the Astrophysical Journal and appears in the online archive arxiv.org.