"The centuries-old quest for other worlds like our Earth has been rejuvenated by the intense excitement and popular interest surrounding the discovery of hundreds of planets orbiting other stars."There is now clear evidence for substantial numbers of three types of exoplanets; gas giants, hot-super-Earths in short period orbits, and ice giants.
"The challenge now is to find terrestrial planets (i.e., those one half to twice the size of the Earth), especially those in the habitable zone of their stars where liquid water and possibly life might exist.
"The Kepler mission, scheduled to launch in 2008, will be a space mission to search for Earth-size and smaller planets around other stars in our neighborhood of the galaxy. Scientists also expect to detect larger planets during the mission. Kepler is a special-purpose spacecraft that precisely measures the light variations from distant stars, looking for planetary transits.
"When a planet passes in front of its parent star, as seen from our solar system, it blocks the light from that star. This is known as a 'transit.' Searching for transits of distant 'Earths' is like looking for the drop in brightness when a moth flies across a searchlight. Measuring repeated transits, all with a regular period, duration and change in brightness, provides a rigorous method for discovering and confirming planets and their orbits – planets the size of Earth and smaller in habitable zones around other stars. Scientists define habitable zones as volumes in space where planets could be that may well have liquid water on their surfaces.
"Kepler will survey four classes of stars – F stars (bigger and brighter than Earth's sun), G stars (similar to our sun in brightness and size) and K and M stars (smaller and less bright than our sun).
"Kepler will operate for four years to view an area of the sky 500 times the area of Earth's moon. During this time, Kepler will continuously monitor the brightness of 100,000 stars in the Milky Way galaxy. The Kepler spacecraft consists of a spacecraft bus and a single instrument called a photometer, that is, a light meter, which can simultaneously measure the brightness variations of stars with a precision of about 20 parts per million.
"Kepler must measure at least three transits of a planet to consider it a valid planetary candidate. Then, each star with a candidate planet must be observed by ground-based telescopes to eliminate any that have nearby stars that produce a signal that imitates a planetary transit. Consequently, the announcement of planetary discoveries can only be made after a minimum of several months. For Earth-size planets in orbit around stars similar to our sun, we must wait a minimum of three years to get three transits plus the ground-based observing time. Thus, the most valuable discoveries cannot be announced until near the end of the Kepler mission in 2012.
"According to researchers, the results will tell scientists how often planets occur in the habitable zone of other stars. If these planets are common, then hundreds of Earth-size planets in the habitable zone and thousands outside the habitable zone will be detected, according to Kepler scientists.
"The Kepler Mission, NASA Discovery mission #10, is specifically designed to survey our region of the Milky Way galaxy to discover hundreds of Earth-size and smaller planets in or near the habitable zone and determine how many of the billions of stars in our galaxy have such planets.
"Results from this mission will allow us to place our solar system within the continuum of planetary systems in the Galaxy.
"Kepler Mission Scientific Objective
"The scientific objective of the Kepler Mission is to explore the structure and diversity of planetary systems. This is achieved by surveying a large sample of stars to:
- "Determine the percentage of terrestrial and larger planets there are in or near the habitable zone of a wide variety of stars
- "Determine the distribution of sizes and shapes of the orbits of these planets
- "Estimate how many planets there are in multiple-star systems
- "Determine the variety of orbit sizes and planet reflectivities, sizes, masses and densities of short-period giant planets
- "Identify additional members of each discovered planetary system using other techniques, and
- "Determine the properties of those stars that harbor planetary systems.
"The Transit Method of Detecting Extrasolar Planets
"When a planet crosses in front of its star as viewed by an observer, the event is call a transit. Transits by terrestrial planets produce a small change in a star's brightness of about 1/10,000 (100 parts per million, ppm), lasting for 2 to 16 hours. This change must be absolutely periodic if it is caused by a planet. In addition, all transits produced by the same planet must be of the same change in brightness and last the same amount of time, thus providing a highly repeatable signal and robust detection method.
"Once detected, the planet's orbital size can be calculated from the period (how long it takes the planet to orbit once around the star) and the mass of the star using Kepler's Third Law of planetary motion. The size of the planet is found from the depth of the transit (how much the brightness of the star drops) and the size of the star. From the orbital size and the temperature of the star, the planet's characteristic temperature can be calculated. From this the question of whether or not the planet is habitable (not necessarily inhabited) can be answered.
"The Kepler Mission Design
"For a planet to transit, as seen from our solar system, the orbit must be lined up edgewise to us. The probability for an orbit to be properly aligned is equal to the diameter of the star divided by the diameter of the orbit. This is 0.5% for a planet in an Earth-like orbit about a solar-like star. (For the giant planets discovered in four-day orbits, the alignment probability is more like 10%.) In order to detect many planets one can not just look at a few stars for transits or even a few hundred. One must look at thousands of stars, even if Earth-like planets are common. If they are rare, then one needs to look at many thousands to find even a few. Kepler looks at 100,000 stars so that if Earths are rare, a null or near null result would still be significant. If Earth-size planets are common then Kepler should detect hundreds of them.
"Considering that we want to find planets in the habitable zone, the time between transits is about one year. To reliably detect a sequence one needs four transits. Hence, the mission duration needs to be at least three and one half years.
"The Kepler instrument is a specially designed 0.95-meter diameter telescope called a photometer or light meter. It has a very large field of view for an astronomical telescope —105 square degrees— or about the area of both your hands held at arm's length, in order to observe the necessary large number of stars. It stares at the same star field for the entire mission and continuously and simultaneously monitors the brightnesses of more than 100,000 stars for the life of the mission.
"The Kepler Space Telescope, which launched earlier this year to find Earth-like planets elsewhere in our galaxy, showed it’s open for business with NASA’s announcement that an exoplanet we thought we knew is like nothing we’ve seen before."Peering at a large planet orbiting very close to its star, HAT-P-7b, the telescope delivered what one scientist called “exquisite” data, proving that it’s ready to start looking for Earth-sized planets in the habitable zones around stars.
"Analyzing just 10 days of data, the scientists even turned up a major surprise: HAT-P-7b isn’t like Jupiter at all. It has a “dark” side and its atmosphere could be made of relatively exotic chemicals like titanium oxide.
“This planet is not like anything in Earth’s solar system,” said Sara Seager, an exoplanetary scientist at MIT.
"The hottest spot on Hat-P-7b could be more than 1,300 degrees Fahrenheit warmer than the coldest spot. There is no comparable planet around our star.
"Kepler’s first findings, some of which were published in the journal Science, augur a new level of exoplanetary research, Seager said.
“This exquisite data is just the tip of the iceberg,” she said. “We’re going to see a new world of exoplanet exploration where discoveries will come much more rapidly than they’ve come in the last 10 years.”
"But while we’ll be learning more about planets in the universe every month, Kepler’s ultimate mission — finding Earth-like planets in the habitable zone — is still going to take years.
"The habitable zone is the term astronomers use to denote the distance from a star where orbiting planets wouldn’t get fried or frozen. In those locations, liquid water could exist — and that’s something that is necessary for life as we know it. In our solar system, Venus, Earth, and Mars lie within the zone. The more planets that Kepler finds in its survey of 100,000 stars, the more likely that Earth-like planets exist in large numbers throughout the universe. And the more Earths out there, the better our chances get of finding life outside our solar system.
"The announcement that Kepler is working as designed is great news for the exoplanetary research community, which had been eagerly awaiting the first data from the instrument. 225993main_kepler-browse
“It appears Kepler is working at its design precision — the noise level of just 40 parts per million per data point is amazing!” Matthew Muterspaugh, an exoplanet researcher at Tennessee State University, wrote in an email to Wired.com.
"Kepler’s mission is primarily statistical. It will tell us how many solar systems like ours likely exist in the universe, but the stars in its field of vision are too far away to characterize in much detail. Kepler will be able to tell us that the planet is in the right place, but not whether it’s suitable for or home to life.
“[Kepler] will be able to say it’s a rocky planet like Venus or Earth, but won’t be able to say whether it’s more like Venus or Earth,” Muterspaugh said.
"So, while Kepler will help sort out how likely life is around the universe, it won’t actually find any definitive signs of it. The stars that it’s looking at are just too far away.
“The downside to the Kepler technique is that the stars tend to be very far away and faint,” said Muterspaugh. “This set of stars will not be well optimized for the follow up observations.”
"The ultimate quest to find other planets that support life will require a series of other missions and telescopes, which focus on planets much closer to home.
"First, we’ll have to find out which local stars have rocky planets. NASA’s first potential mission with that capability is the SIM Lite Astrometric Observatory. It will be able to find Earth-sized planets around 60 stars in our neighborhood.
"Once these planets are identified by SIM or other missions like Gaia and CoRoT, NASA will be able to peer at them with the powerful James Webb Space Telescope or a dedicated planet-hunting telescope like the agency’s proposed Terrestrial Planet Finder missions.
"For planets larger than Earth, the James Webb telescope will be able to characterize them effectively, but for the true Earths, we’ll need the more powerful instrumentation of the TPF. It will be a long road from finding the first Earth-like planet with Kepler to knowing how habitable any extrasolar planet is, but if all goes well, scientists think we could have answers by 2020.
“The timeline is set for in 2020 having that image of our brother world out there,” Muterspaugh said. “I think that this is a fantastic thing for our generation to do, answering a question that’s thousands of years old: are we alone?”
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