Nearest 'Second Earth' May be Right Next Door
David A. Aguilar (CfA)
This artist's conception shows a hypothetical habitable planet with two moons orbiting a red dwarf star.
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.
The nearest habitable planet beyond the solar system may be relatively close to Earth, though the parent star would be a cooler and redder than the sun -- an interesting implication for any indigenous life.
Extrapolating from findings by NASA's planet-hunting Kepler Space Telescope, scientists on Wednesday said roughly six percent of so-called red dwarf stars have Earth-sized planets properly positioned around their parent stars so that liquid water could exist on their surfaces.
Water is necessary for life -- at least life as we know it.
While Kepler's prime mission is to find Earth-sized worlds around sun-like stars, its observations of red dwarf stars are providing additional food for thought, particularly because red dwarfs are by far the most common type of star in the galaxy. Typically, these stars are about one-third the size of the sun and about 1,000 times dimmer. Three out of every four stars in the galaxy are red dwarfs, adding up to about 75 billion.
At least one red dwarf has been determined to host a planet that is roughly twice the size of Earth.
"We decided that it would make sense to see if we could look at the red dwarfs in Kepler (data) whether we would find the occurrence of planets would be consistent," with the earlier study, astronomer Courtney Dressing, with the Harvard-Smithsonian Center for Astrophysics, told Discovery News.
The team looked at 95 candidate planets circling red dwarf stars observed by Kepler and found that at least 60 percent have planets smaller than Neptune. Most were not the right size or temperature to be Earth-like, but three were found to be both warm and approximately Earth-sized. Scientists, however, do not have enough information to assess if they are rocky worlds, like Earth.
Nevertheless, statistically that would mean six percent of all red dwarf stars should have a Earth-sized planet, Dressing said, adding that since 75 percent of the closest stars are red dwarfs, the nearest Earth-like world may be just 13 light-years away.
A light year is the distance that light, which moves at about 186,000 miles per second, can travel in one year -- roughly 6 trillion miles, a relative stone's throw in cosmic scales.
"If we're looking for a planet like the Earth that could possibly support life, we don't need to look as far as we thought we did initially," Dressing said.
When it's young, a red dwarf star frequently erupts with strong ultraviolet flares as shown in this artist's conception.David A. Aguilar
It will take more sensitive telescopes than what exist today to uncover telltale chemical signs of life, if any exist, but that doesn't stop scientists from imagining.
Plants, for example, may be very different on a planet that evolved around a red dwarf star, rather than a yellow star like the sun. Not only would the wavelengths of light be redder, but the planets would be exposed to more radiation over the long haul, conditions that might make for dark brown and black plants, rather than green, said astronomer Lisa Kaltenegger, with the Max-Planck Institute for Astrophysics in Germany.
"The other question is, this is speculative of course, but would plant life if it came along, also use chlorophyll, or would they develop something completely different?" Kaltenegger told Discovery News. "Plants on Earth are extremely evolved to use the energy from the sun. You could envision different schemes that do something similar."
Taking the speculation a bit farther, Kaltenegger said she wouldn't expect much difference in any microbial life on red dwarf planets.
"We don't know if life actually develops, but one big difference is the amount of radiation (on red dwarf planets.) The argument people make is that it would be really bad for life on the surface. But if you evolve for that, you shouldn't have any problem," she said.
Dressing's findings, which were announced at a press conference on Wednesday, are expected to be published in an upcoming edition of The Astrophysical Journal.