After carefully measuring the masses of small, rocky exoplanets, astronomers have come to the conclusion that our solar system may not be unique and, by extension, those exoplanets surveyed are likely composed of similar materials as Earth.
Using the High-Accuracy Radial velocity Planet Searcher (HARPS) North instrument on the 3.6-meter Telescopio Nazionale Galileo in the Canary Islands, the researchers, led by Courtney Dressing of the Harvard-Smithsonian Center for Astrophysics (CfA), focused on the slight wobble small exoplanets exert on their host stars.
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As an exoplanet orbits its star, the tiny gravitational pull can tug the star slightly off-center. By measuring this "wobble" (through the analysis of the shifting of wavelength of the detected starlight), astronomers are able to determine the mass of that exoplanet. This mode of exoplanet detection is known as the "radial velocity" method. By combining these mass measurements with observations made by NASA's Kepler Space Telescope, which uses the "transit method" to detect exoplanets and determine their physical size, the density of that exoplanet can be calculated. With this valuable information, scientists can begin to understand what that world is made of.
In a new study presented today (Jan. 5) at the 225th American Astronomical Society meeting in Seattle, Wash., and accepted for publication in the The Astrophysical Journal, Dressing's team discussed their findings for small worlds that have a lot more in common with Earth than we thought.
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"Our strategy for using HARPS-North over the past year has been to focus on planets less than two times the diameter of Earth and to study a few planets really well," said astronomer David Charbonneau, also of the CfA and head of the HARPS-North Science Team.
One key discovery focused on Kepler-93b, an exoplanet identified by Kepler. It is known to be 1.5 times the size of Earth, but it has an extreme 4.7-day orbit around its host star. Before Dressing's team began measurements of the distant world, very little was known about its mass and composition. But HARPS-North was able to precisely measure its mass to 4.02 times the mass of Earth. From this information alone, the astronomers could say that this world was a rocky exoplanet.
Then, the astronomers measured the densities of "all ten known exoplanets with a diameter less than 2.7 times Earth's," according to a CfA press release, and found that five of those worlds with diameters less than 1.6 times that of Earth showed a very tight relationship between mass and size. Most interestingly, both Earth and Venus, when added to the analysis, fitted neatly into the same correlation.
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As we know that Earth has a rocky composition with an iron core (and Venus is thought to possess similar characteristics), perhaps these planets, orbiting different stars, also have the same composition. There are a few exoplanetary exceptions, but for the most part, small worlds closely orbiting their stars have some very terrestrial characteristics.
"To find a truly Earth-like world, we should focus on planets less than 1.6 times the size of Earth, because those are the rocky worlds," said Dressing.
As a fun aside, the CfA astronomers created a recipe for how you could "bake" your very own Earth-like model world:
Ingredients: 1 cup magnesium, 1 cup silicon, 2 cups iron, 2 cups oxygen, 1/2 teaspoon aluminum, 1/2 teaspoon nickel, 1/2 teaspoon calcium, 1/4 teaspoon sulfur, dash of water delivered by asteroids.
Blend well in a large bowl, shape into a round ball with your hands and place it neatly in a habitable zone area around a young star. Do not over mix. Heat until mixture becomes a white hot glowing ball. Bake for a few million years. Cool until color changes from white to yellow to red and a golden-brown crust forms. It should not give off light anymore. Season with a dash of water and organic compounds. It will shrink a bit as steam escapes and clouds and oceans form. Stand back and wait a few more million years to see what happens. If you are lucky, a thin frosting of life may appear on the surface of your new world.
Sounds like the kind of world I'd want to bake...
Source: CfA
How do you make an Earth-like planet? The "test kitchen" of Earth has given us a detailed recipe, but it wasn't clear whether other planetary systems would follow the same formula. Now, astronomers have found evidence that the recipe for Earth also applies to terrestrial exoplanets orbiting distant stars
Cowboys & Aliens are Coming!
July 29, 2011 --
If aliens are going out of their way to kick up dust in the Wild West, as they do in the upcoming movie "Cowboys & Aliens," they must be coming from somewhere. Life could take root on a moon or a meteorite. But to nurture the kind of life that could destroy our saloons and harass our livestock, a planet might be the most suitable. So far, Kepler, a NASA orbiting telescope that searches for planets beyond our solar system, has detected over 1,200 exoplanets. Surely there must be a few candidates among this group that could meet some of the most basic requirements to host life? Explore some far-out worlds that could support aliens, be they cattle-rustling characters or a more peaceful people.
The Basics
First, let's lay out some basic criteria. Kepler hasn't identified many rocky worlds and a solid surface is essential for life to take root. Size matters: The mass of the planet helps astrophysicists infer what it's made of. Some planets are Earth-sized. Others are several times the size of our planet. And then there are gas giants, which can range from "Neptune sized" to "super-Jupiters." Orbit: To support life, a planet must be in a stable orbit around its star -- no planets with wonky orbits that will eventually dump them into their star for a fiery death. Goldilocks Zone: This is a region not too hot or too cold that gives the planet enough distance from its parent star to have liquid water, key for life. Loner Stars: Single stars make better parents. In 2010, a pair of closely orbiting binary stars was spotted surrounded by what could be the debris of former planets. Unknowns: Some factors for life can't be confirmed one way or the other from the data available about extrasolar planets. These include: water, chemical compounds such as ammonia; a nitrogen-rich atmosphere; a magnetic field to repel solar and cosmic radiation; and more. BUT, some planets do have a head-start, beginning with Gliese 581D.
Gliese 581d
Located a mere 20 light-years away, practically our backyard in cosmic terms, Gliese 581d is situated on the "outer fringes" of the Goldilocks zone, orbiting a red dwarf star. The planet may be warm enough and wet enough to support life in much the same manner as Earth. It might also contain a thick carbon atmosphere. If we ever need a new Earth and have the means to get there, Gliese 581d may be our best bet for now.
Gliese 581g
When it was first detected and reported last year in Astrophysical Journal, Gliese 581g appeared to be the perfect candidate for a true "Earth-like" planet. Located in the same star system as Gliese 581d (and detected earlier), Gliese 581g seemed to be the right size and located within a habitable zone away from its parent star. Gliese 581g was said to have three times the mass of Earth, making it possible for the planet to hold an atmosphere. However, since its discovery, follow-up studies have alleged that Gliese 581g might have been a false alarm. In other words, the planet might not exist at all.
GJ 1214b
Dubbed a "waterworld" and located a mere 42 light-years from Earth, GJ 1214b orbits near a red dwarf star about one-fifth the size of our sun. What makes this planet unique is that it appears to be primarily composed of water, although GJ 1214b is 6.5 times the mass of Earth and 2.7 times wider, which classifies it as a "super-Earth." This planet also has a steamy atmosphere composed of thick, dense clouds of hydrogen, which, although it might not the case with this planet, could incubate life.
HD 209458b
Situated 150 light-years from Earth, HD 209458b is a planet that holds traces of water vapor in its atmosphere, and also contains basic organic compounds that, on Earth, foster the development of life. But there are two factors working against HD 209458b as a suitable habitat. The planet is very hot due to its close proximity to its parents star, and it's a gas giant, so no solid surfaces.
Kepler-10b
If Kepler-10b were located further from its parent star, it might have had a chance of hosting life. Kepler-10b was the first "iron-clad proof of a rocky planet beyond our solar system" back in 2001. It was even dubbed the "missing link" of extrasolar planetary research. When it comes to the search for life, though, Kepler 10-b is missing a lot of other ingredients -- just minor things like water or an atmosphere.
Project Icarus
When venturing to a new star system to explore the possibility of extraterrestrial life, trying a star that has already shown itself to nurture planets -- even if they're not the kind you're looking for -- could be a promising strategy. Project Icarus, an ambitious five-year study into launching an unmanned spacecraft to an interstellar destination, has identified two stars located within 15 light-years that might fit the bill: "epsilon Eridani, a single K star 10.5 light-years away, and the red dwarf GJ 674, 14.8 light-years away." Indirect evidence has also shown that epsilon Eridani may already hold smaller worlds scientists simply haven't detected yet. Also, red dwarf star systems generally may be a safe haven for life.
Are We Alone?
Taking into account the number of exoplanets that have been detected, as well as the vastly greater number that are estimated to be out there, some astrophysicists are convinced that extraterrestrial life is inevitable. After all, the Milky Way may be loaded with as many as 50 billion alien worlds. Some even think we'll find alien life by 2020. Others, however, say it may not exist at all. Recently, astrophysicists David Spiegel of Princeton University and Edwin Turner from the University of Tokyo suggested we might be alone in the universe, based on their interpretation of the Drake equation, a formula meant to determine loosely the probability of the existence of life beyond Earth. According to their analysis, just because life on Earth took shape early, endured and prospered doesn't mean the same process would naturally and inevitably occur elsewhere in the universe. Discovering life elsewhere, however, would be the only means of settling this debate. Unless the aliens find us first, of course.