Detecting an exoplanet

If a planet passes directly between a star and an observer's line of sight, it blocks out a tiny portion of the star's light, thus reducing its apparent brightness. Sensitive instruments can detect this periodic dip in brightness. From the period and depth of the transits, the orbit and size of the planetary companions can be calculated. Smaller planets will produce a smaller effect, and vice versa. A terrestrial planet in an Earth-like orbit, for example, would produce a very small dip in stellar brightness that would last just a few hours. This is one of several ways in which astronomers can find exoplanets.

Other-worldly pursuit

In the basement of the astronomy department at the University of California-Berkeley, a half-dozen researchers sit behind computers three or four nights each month and look for planets beyond our solar system. Using an internet interface and a video link, they remotely control one of the largest telescopes in the world—a 270-ton instrument at the W.M. Keck Observatory in Hawaii, where two round telescopes sit perched on the summit of Mauna Kea like eight-story snowballs.

As the astronomers analyze the starlight data collected by Keck, they look for repetitious shifts in the wavelengths of individual stars. Those shifts could mean a star is being orbited by its own planet.

"There's no sort of magic time—you know, a eureka moment—when suddenly, 'Ah-hah, it's there!'" says Andrew Howard, a member of the Berkeley team, explaining how a star suspected of hosting a planet must be measured over and over. "It's pretty exciting when you discover that there's a very good candidate: 'Wow! If this thing pans out it'll be the second smallest planet ever discovered!'"

The search for extrasolar planets—or exoplanets—is now one of the hottest pursuits in astronomy. Since the existence of exoplanets was confirmed in the 1990s, astronomers have tallied over 400 of them. It's a number that's growing quickly as planet-hunting technology expands and astronomers endeavor to find something out there comparable to Earth.

Howard's candidate did pan out: After magnifying a star inconveniently named HD156668 about 90 separate times over five years, the Berkeley team concluded it was being orbited by a planet about four times as heavy as the Earth—the second smallest exoplanet known to orbit a star similar to our sun. Howard's research team announced the discovery at the annual meeting of the American Astronomical Society (AAS) in January.

It was a significant discovery because of the technical difficulty of finding smaller exoplanets (it was "a nice sort of trophy" for the team, said Howard), and also because most exoplanets found so far have been comparable to the size of Jupiter, at 300 times the mass of the Earth, or even larger.

But finding an Earth-size planet is the gem many exoplanet hunters are looking for, and it's the chief goal of NASA's Kepler mission, which announced its own discovery of five new exoplanets at January's AAS meeting. Launched from Florida's Cape Canaveral in March of last year, the cylinder-shaped Kepler spacecraft orbits the sun rather than the Earth, and stares persistently into a single swath of space about the size of your hand at arm's length. It monitors the glow of 156,000 stars and beams information home to four clusters of computers, which take note of any star that suddenly dims by a small amount—perhaps 1 percent—then brightens again several hours later.

That light pattern may be the signature of an exoplanet dozens or hundreds of light-years away. It works like this: If the plane of an exoplanet's orbit lies edge-on toward us, the planet will pass in front of its star, blocking a portion of the light, like a moth near a lamp. The dimming will occur at regular intervals, each time the planet orbits the star, perhaps every few days—or every few years.

Technically, Kepler's method for detecting exoplanets is different from the Berkeley team's method, which measures fluctuations in wavelength rather than brightness. But astronomers combine the methods to confirm their data and get an accurate calculation of an exoplanet's diameter and mass, which reveals its density. Kepler's principal science investigator, William Borucki, said knowing the density helps theoreticians understand the structures of exoplanets: "High density? Gonna be something like the Earth. Low density? We're not quite sure what it means, but it's probably Jupiter-like. Something intermediate? Probably like Neptune."

The five exoplanets Kepler discovered last year are "hot Jupiters," so called because of their large size and intense temperatures (2,200°F to 2,900°F). The largest one of them turned out to be the least dense: Its average density was one-fifth that of water, or like "a piece of Styrofoam."

All five of Kepler's exoplanets are so close to their host stars that they complete their orbits every three to five days. Planets with longer orbital periods will take more of Kepler's time: Finding an exoplanet with an orbit similar to Earth's, for instance, requires three years of observation, since the planet must pass before its star three times before its discovery can be confirmed. Borucki said Kepler has identified hundreds of potential planets, but many will turn out to be false alarms, caused by other phenomena such as eclipsing binary stars.

The total number of exoplanets in space is a risky guess, but if each star in the Milky Way were orbited by just one planet, the galaxy would contain over 100 billion.

Popular interest in the question of life beyond our solar system is helping fuel the hunt for smaller and smaller exoplanets. Astronomers are looking for Earth-size planets that may lie within the "habitable zone" of their stars—an orbital distance where water can physically exist in its liquid form.

Jason Lisle, an astrophysicist who works as a research scientist for Answers in Genesis, a creationist organization, sounded upbeat about the hunt for exoplanets and said he expected improving technology to discover smaller planets. But, "I would not expect there to be another planet out there that's like Earth in the sense of having all the right ingredients for life, and having life on its surface. And so far, observations have borne out that prediction."

Lisle noted that some planetary systems—such as those with Jupiter-size planets orbiting close to their stars—or one discovered last year with a planet orbiting in the opposite direction of its star's rotation—throw a wrench in current theories about the formation of stars and planets.

Researchers have yet to find an exoplanet that can properly be called "Earth-like." Or hospitable. The smallest exoplanet discovered so far, CoRoT-7b, is 70 percent larger than Earth and has a density similar to Earth or Mercury, most likely making it a "rocky" planet. But with surface temperatures reaching perhaps 3,600°F, "lava" planet is a more realistic description.

Kepler has found even warmer and stranger worlds: "We've got a companion that is planet-sized, but is much hotter than the star it orbits," said Borucki. The star is about 16,500°F and the companion is 21,000°F, more than double the temperature of our sun. Borucki speculates the companion could be something like a white dwarf star, but he says, "We're a little bit puzzled about some of these things too."

But the Kepler mission itself is proof that perseverance precedes discovery. Borucki pioneered the idea of measuring star brightness to detect exoplanets in the '80s, and throughout the '90s tried to convince NASA to put a special telescope in space for that purpose. NASA rejected the idea as technologically impractical for several years. "And we were laughed at for a very long time," says Borucki. But by 2000 his team had overcome the technical challenges and sold the space agency on Kepler.

"What we're talking about is a major effort by humankind. You do it one step at a time," said Borucki of the extrasolar planet search. "The technology simply didn't exist 10, 15 years ago to do these things."

Source:- http://www.worldmag.com/articles/16409

Exoplanetary Atmosphere Reveals Glowing Methane

In a groundbreaking, new find, astronomers announce that they've managed to identify glowing methane gas in the atmosphere of an exoplanet. The finding has been made using a ground-based observatory, as opposed to an orbit-based one, so experts believe that many more such discoveries could be in store for us over the coming years. The team behind the findings also says that, if its predictions turn out to be accurate, such methane-related events may actually be a common occurrence around exoplanets.

The new investigation was conducted at the NASA Jet Propulsion Laboratory (JPL), in Pasadena, California. The JPL science group was led by expert Mark Swain, who revealed that the exoplanet that was the target of the new investigation was one of the first such bodies to have had their atmosphere measured via spectrometry, in previous studies. The exoplanet is known as HD 189733b, and astronomers have already detected carbon dioxide, methane and water vapors in its atmosphere. However, the new study revealed glowing methane, which means that the gas may be resembling the Earth's auroras, the group adds.

Researchers underline the fact that this discovery should have not come like a total surprise. “Indeed, finding glowing methane in the atmosphere of a planet that is tidally-locked with its parent star is unusual, but the instance is not rare. It's not particularly surprising since we have seen fluorescent methane in Jupiter, Saturn and even Titan,” Wesleyan University expert Seth Redfield explains. He has not been involved in the new study, but has accompanied the paper with a commentary. Both pieces were published in the February 4 issue of the esteemed scientific journal Nature.

But the main question on everyone's minds at this point is where this methane originates from. On Earth, it can come from natural phenomena such a volcanism, but it is also being produced in large amounts by animals and microorganisms. Experts now plan to solve this mystery by using more advanced, Earth-based telescopes. Unlike their orbit-based counterparts, these instruments are a lot larger, and can detect more details in their targets. However, they can only conduct accurate observations in certain wavelengths, as most of the spectrum is being absorbed or scattered by the planet's atmosphere,Space reports.

New Technique Can Identify Molecules on Exoplanets

Researchers at the American space agency NASA have recently announced that they managed to create a new method of using modest, ground-based telescopes for producing amazing new science. The technique was just recently used on an exoplanet some 63 light-years away, with the 30-year-old, 3-meter-diameter (10-foot) NASA Infrared Telescope Facility, atop Mauna Kea, in Hawaii. The method makes it easy to identify specific molecules in the atmosphere of exoplanets, as evidenced by the fact that the small telescope was able to find an organic molecule around the Jupiter-size planet it observed. “The fact that we have used a relatively small, ground-based telescope is exciting because it implies that the largest telescopes on the ground, using this technique, may be able to characterize terrestrial exoplanet targets,” says Mark Swain, the leader of the new investigation, and a scientist at the NASA Jet Propulsion Laboratory (JPL), in Pasadena, California. Details of this work appear in the February 3 issue of the esteemed scientific journal Nature. Swain was lead author of the paper. “Given favorable observing conditions, this work suggests we may be able to detect organic molecules in the atmospheres of terrestrial planets with existing instruments,” he adds, stating that the ITF is ranked only 40th in a top of the world's ground-based telescopes, which carries considerable implications for the possibilities offered by the other 39 larger facilities. The data collected with the ITF provided new info on the exoplanet's atmospheric composition and conditions, something that has never before been achieved using a telescope on the ground. The organic molecules detected around the planet HD189733b belonged to methane and carbon, the JPL team reports. Such a discovery was made possible through the use of a new calibration method that is capable or removing systematic observation errors from the picture. These errors are produced as light coming from the target passes through our planet's atmosphere. Further mistakes get passed on as the telescope moves during observations, in tune with the planet's motion. “As a consequence of this work, we now have the exciting prospect that other suitably equipped yet relatively small ground-based telescopes should be capable of characterizing exoplanets. On some days we can't even see the sun with the telescope, and the fact that on other days we can now obtain a spectrum of an exoplanet 63 light-years away is astonishing,” concludes ITF support scientist John Rayner. He is also the NASA expert who built the SpeX spectrograph, the instrument used for the new measurements.

Probing Exoplanets from the Ground: A Little Telescope Goes a Long Way

This artist concept shows the planetary system called HD 189733, located 63 light-years away in the constellation Vulpecula. Astronomers performed the surprising feat of measuring methane and carbon dioxide in the atmosphere of the system's known gas planet from a telescope on the ground -- NASA's 3-meter (6-foot) Infrared Telescope Facility, located atop Mauna Kea in Hawaii. (Credit: NASA/JPL-Caltech)

ScienceDaily (Feb. 9, 2010) — NASA astronomers have successfully demonstrated that a David of a telescope can tackle Goliath-size questions in the quest to study Earth-like planets around other stars. Their work, reported in the journal Nature, provides a new tool for ground-based observatories, promising to accelerate by years the search for prebiotic, or life-related, molecules on planets orbiting stars beyond our solar system.

The scientists reported on a new technique used with a relatively small Earth-based telescope to identify an organic molecule in the atmosphere of a Jupiter-size planet nearly 63 light-years away. The measurement revealed details of the exoplanet's atmospheric composition and conditions, an unprecedented achievement from an Earth-based observatory.

The surprising new finding comes from a venerable 30-year-old, 3-meter-diameter (10-foot) telescope that ranks 40th among ground-based telescopes -- NASA's Infrared Telescope Facility atop Mauna Kea, Hawaii.

The new technique promises to further speed the work of studying planet atmospheres by enabling studies from the ground that were previously possible only through a few very high-performance space telescopes. "Given favorable observing conditions, this work suggests we may be able to detect organic molecules in the atmospheres of terrestrial planets with existing instruments," said lead author Mark Swain, an astronomer at NASA's Jet Propulsion Laboratory, Pasadena, Calif. This can allow fast and economical advances in focused studies of exoplanet atmospheres, accelerating our understanding of the growing stable of exoplanets.

"The fact that we have used a relatively small, ground-based telescope is exciting because it implies that the largest telescopes on the ground, using this technique, may be able to characterize terrestrial exoplanet targets," Swain said.

Currently, more than 400 exoplanets are known. Most are gaseous like Jupiter, but some "super-Earths" are thought to be large terrestrial, or rocky, worlds. A true Earth-like planet, with the same size as our planet and distance from its star, has yet to be discovered. NASA's Kepler mission is searching from space now, and is expected to find several of these earthly worlds by the end of its three-and-a-half-year prime mission.

On Aug. 11, 2007, Swain and his team turned the infrared telescope to the hot, Jupiter-size planet HD 189733b in the constellation Vulpecula. Every 2.2 days, the planet orbits a K-type main sequence star slightly cooler and smaller than our sun. HD189733b had already yielded breakthrough advances in exoplanet science, including detections of water vapor, methane and carbon dioxide, using space telescopes. Using the new technique, the astronomers successfully detected carbon dioxide and methane in the atmosphere of HD 189733b with a spectrograph, which splits light into its components to reveal the distinctive spectral signatures of different chemicals. Their key work was development of a novel calibration method to remove systematic observation errors caused by the variability of Earth's atmosphere and instability due to the movement of the telescope system as it tracks its target.

"As a consequence of this work, we now have the exciting prospect that other suitably equipped yet relatively small ground-based telescopes should be capable of characterizing exoplanets," said John Rayner, the NASA Infrared Telescope Facility support scientist who built the SpeX spectrograph used for these measurements. "On some days we can't even see the sun with the telescope, and the fact that on other days we can now obtain a spectrum of an exoplanet 63 light-years away is astonishing."

In the course of their observations, the team found unexpected bright infrared emission from methane that stands out on the day side of HD198733b, indicating some kind of activity in the planet's atmosphere. Swain said this puzzling feature could be related to the effect of ultraviolet radiation from the planet's parent star hitting the planet's upper atmosphere, but more detailed study is needed. "This feature indicates the surprises that await us as we study exoplanet atmospheres," he added.

"An immediate goal for using this technique is to more fully characterize the atmosphere of this and other exoplanets, including detection of organic and possibly prebiotic molecules" like those that preceded the evolution of life on Earth, said Swain. "We're ready to undertake that task." Some early targets will be the super-Earths. Used in synergy with observations from NASA's Hubble, Spitzer and the future James Webb Space Telescope, the new technique "will give us an absolutely brilliant way to characterize super-Earths," Swain said.

Other authors are Pieter Deroo, Gautam Vasisht and Pin Chen of JPL; Caitlin A. Griffith of the University of Arizona, Tucson; Giovanna Tinetti of University College London; Ian J. Crossfield of UCLA; Azam Thatte of the George Institute of Technology, Atlanta; Jeroen Bouwman, Cristina Afonso and Thomas Henning of Max-Planck Institute for Astronomy, Heidelberg, Germany; and Daniel Angerhausen of the German SOFIA Institute, Stuttgart, Germany.

The work was carried out with funding from NASA's Office of Space Science in Washington, D.C. The NASA Infrared Telescope Facility is managed by the University of Hawaii's Institute for Astronomy. JPL is managed by the California Institute of Technology for NASA.

Mark R. Swain, Pieter Deroo, Caitlin A. Griffith, Giovanna Tinetti, Azam Thatte, Gautam Vasisht, Pin Chen, Jeroen Bouwman, Ian J. Crossfield, Daniel Angerhausen, Cristina Afonso & Thomas Henning. A ground-based near-infrared emission spectrum of the exoplanet HD 189733b. Nature, 2010; 463 (7281): 637 DOI:10.1038/nature08775