Star's Wobble Could Reveal Nearby 'Earth-Like' Exoplanet
Computer analysis of the wobble of a nearby star may have turned up evidence for a small, Earth-sized world -- but don't get too excited about it being habitable.
Starlight contains a lot of information.
By studying the electromagnetic spectrum of a star's light, you can see what elements it contains. You can also deduce its age, mass, stability and spin. As astronomical techniques and technologies have become more sophisticated, alien planets that would have otherwise remained invisible can also be detected via their gravitational tug on their host star.
This mode of exoplanetary detection is known as the "radial velocity method" and it depends on the analysis of the periodic shift in the frequency of starlight to reveal the gravitational fingerprint of orbiting worlds. Basically, by watching a star's light for a long enough period, astronomers can see a star's "wobble," a sure sign that a planet - or a system of planets - is in tow.
Now, a team of astronomers, led by Suman Satyal of the University of Texas at Arlington, has delved into the starlight of a nearby red dwarf star already known to possess two exoplanets, revealing there's the potential for a third exoplanet, possibly as small and as rocky as Earth, sandwiched between the orbits of the two known worlds.
Gliese 832 is a well-known red dwarf. With a mass around half that of our sun, this diminutive star, located only 16 light-years away, has two exoplanets called Gliese 832b and Gliese 832c. Gliese 832b is the larger of the two and has the widest orbit, located 3.53 AU from its host star. It is also more massive, "weighing in" at around 60 percent the mass of Jupiter. Gliese 832c on the other hand is classified as a "super-Earth" of around five times more massive than Earth. It's orbit is extremely compact, coming within 0.16 AU of its star. As a comparison, in our solar system, the innermost planet Mercury comes no closer than 0.3 AU to the sun.
Gliese 832c hit the headlines in 2014, lauded as a possible "Earth 2.0." Though this is certainly a possibility, according to planetary scientists, it is more likely to be a hostile "Venus 2.0″ with a thick, life-choking atmosphere.
Both 832b and 832c were detected by astronomers watching the star's light frequency slightly oscillate, an effect known as Doppler Shift. Much in the same way we hear a higher-pitch siren as a police car approaches compared to when the police car drives away, as a planet's gravity pulls a star toward us, its wavelength will become more compressed (increasing in frequency). As the planet orbits away, the star will also be pulled away, increasing the light's wavelength (decreasing the frequency). Through computer analysis of these oscillations, astronomers can "see" the orbits of planets around stars without actually seeing the planets themselves. Within these radial velocity measurements the companion planets' masses, orbital periods and orbital distances can be deduced by using established Keplerian laws of planetary motion.
Now, by revisiting the Gliese 832 star system, Satyal's team has taken a high-resolution look at the radial velocity data from the star and used computer modeling to see if another exoplanet "fits" between the orbits of 832b and 832c.
"We obtained several radial velocity curves for varying masses and distances for the middle planet," they write in a paper published by the arXiv pre-print service.
Their analysis reveals that another exoplanet could indeed exist with an orbit between 0.25 to 2.0 AU from the star with a mass of 1 to 15 Earth masses. This range is pretty wide, but it provides an invaluable insight for future observations of the star system. An exoplanet within these orbital constraints would be in a stable orbit and would likely be another super-Earth, possibly a world occupying the star's habitable zone.
The habitable zone around any star is the region that is neither too hot or too cold, where water could exist in a liquid state on the planetary surface. As we all know, this is one of the key conditions for life (as we know it) to evolve, hence all the excitement whenever any world is discovered orbiting its star within the habitable zone.
It's worth remembering that Earth orbits the sun at 1 AU, pretty much in the middle of our star's habitable zone. Red dwarfs are much smaller and therefore cooler, so have far more compact habitable zones. Therefore, to maintain water in a liquid state on a hypothetical "Earth-like" planet orbiting a red dwarf, its orbit would have to be far closer. Red dwarfs have often been sited as key locations for alien life to thrive as, by their nature, they are long-lived and may allow complex life to evolve. But red dwarfs are known to be extremely active, often erupting with powerful flares that would irradiate any planet that orbits too close, requiring that planet to have a very well developed natural shielding in the form of a strong magnetosphere.
As you can see, it's one thing modeling the possible presence of a small rocky world around a nearby star, but it is quite another to find a true "Earth-like" planet that could support life as we know it. But it's important to try to at least pull any clues from a star's slight wobble to potentially help us track down alien worlds with any Earth-like quality.
Source: arXiv, h/t Physorg.com
Artistic representation of the potentially habitable exoplanet Gliese 832 c as compared with Earth. Gliese 832 c is represented here as a temperate world covered in clouds. The relative size of the planet in the figure assumes a rocky composition but could be larger for a ice/gas composition.
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.
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.
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.
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.
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.
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.
"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.
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.
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.
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.