People question every day whether or not there are Earth-like planets in our universe, but discovering these possibilities is a very difficult, technical challenge. Finding these terrestrial, Earth-sized planets requires ultra-sensitive detection techniques. Certain methods, such as “astrometry”, “radial velocity” and “optical interferometry”, are better suited for locating massive Jovian planets, so astronomers need techniques such as “microlensing” and “photometry” to scale down to Earth-sized planets and orbits. This site will investigate the various planet-detection practices and focus on those best suited for Earth-sized planets. In addition to the various techniques, the Kepler and CoRoT missions will be explained, two projects that are showing astronomers the potential for finding coveted Earth-like planets.
By the way, feel free to check out the pictures and Youtube videos on the right-hand column. Clicking on each piece will take you right to its individual site.
Tuesday, December 9, 2008
Lesser-Used Methods For Detecting Earth-Sized Planets
Astrometry
Astrometry is the oldest search method for extra solar planets. It consists of measuring a star's position in the sky and observing how that position changes over time. Astronomers use astrometric techniques to try and detect extra solar planets by measuring the gravitational influence that a planet will have over its parent star, causing the star to wobble in a tiny circular or elliptical orbit. Unfortunately for astronomers searching for Earth-sized planets, the astrometric method is most sensitive to planets with large orbits. Only planets with 6.6 Earth masses or greater have been detected, and the ability to find smaller, Earth-sized objects hardly exists. For these reasons, astrometry continues to be a lesser-used technique for finding them.
Optical Interferometry
Optical Interferometry combines the light of multiple telescopes to act as one larger telescope. Interferometry once used radio wavelengths to observe the structure of distant galaxies, but for the last 15 years, astronomers have used optical wavelengths, which are leading to new advances in the search for extra solar planets. Optical interferometry has the potential to provide information on the orbits and masses of extra solar planets of any age or mass. It can make very precise astrometric measurements and can detect the motion of the planet and parent star around their common center of gravity. The Keck Interferometer is a telescope in Hawaii, which is made of two twin telescopes, 85 meters apart from each other. This telescope will study dust clouds around stars in the hope to find the formation of Earth-sized planets.
Radial Velocity
Radial Velocity, also known as Doppler spectroscopy, detects extra solar planets through observations of Doppler Shifts in the spectrum of the star around which the planet orbits. Unfortunately for seekers of Earth-sized planets, this method is best suited for detecting very massive objects close to the parent star. These massive planets cause the largest changes in their radial velocity and are therefore easiest to detect. The chance of observing smaller and more distant planets is increasing, but Earth-sized planets remain undetectable with current techniques. Even so, the hunt for these Earth-sized planets through this technique continues. The High Accuracy Radial Velocity Planet Searcher (HARPS) is currently the best radial velocity instrument available for detecting extra solar planets. HARPS has already detected some Uranus- and Neptune- sized planets with his technique, which represents an important step in the future detection of smaller, potentially Earth-sized extra solar planets.
Microlensing
Microlensing is an event that occurs when an object with enough mass—typically an orbiting star or even a black hole—passes between our line of sight and a background star. The strong gravitational pull of the object bends light rays from the distant star, magnifying that light as a lens would. A person sees the star become brighter as the “lens” passes in front. An extra solar planet will act like a tiny defect on the light curve of the star it is orbiting.
The discovery of OGLE 2005-BLG-390Lb, the first cool rocky/icy extrasolar planet, first brought to mind the high sensitivity of the microlensing technique, because the planet is less than 10 Earth-masses. Now astronomers are using the technique to search for earth-sized planets, and astronomers are hopeful. “Even a signal from an earth-mass planet can be relatively large—tens of percent, which is very detectable,” noted astronomer Scott Gaudi, a pioneer in microlensing techniques.
A microlensing “event” can last days, but the planet’s presence only changes the signal for about one day. Therefore, the data collected needs to be analyzed quickly to notice any possible changes from a planet. The Microlensing Follow Up Network, dubbed MicroFUN, gathers data from observatories around the world to monitor microlensing events around-the-clock. Additionally, an automated anomaly detector, which went into operation for the 2007 microlensing observing season, experiences immediate feedback provided by robotic telescopes.
With such a large effort directed toward microlensing, there is a fair chance of detecting an earth-size planet in the coming years. This could be possible with an intricate network of wide-field telescopes or a space-based telescope. The detection limit of gravitational microlensing extends below 0.1 Earth-masses, which is very promising for finding the coveted earth-sized planets.
The discovery of OGLE 2005-BLG-390Lb, the first cool rocky/icy extrasolar planet, first brought to mind the high sensitivity of the microlensing technique, because the planet is less than 10 Earth-masses. Now astronomers are using the technique to search for earth-sized planets, and astronomers are hopeful. “Even a signal from an earth-mass planet can be relatively large—tens of percent, which is very detectable,” noted astronomer Scott Gaudi, a pioneer in microlensing techniques.
A microlensing “event” can last days, but the planet’s presence only changes the signal for about one day. Therefore, the data collected needs to be analyzed quickly to notice any possible changes from a planet. The Microlensing Follow Up Network, dubbed MicroFUN, gathers data from observatories around the world to monitor microlensing events around-the-clock. Additionally, an automated anomaly detector, which went into operation for the 2007 microlensing observing season, experiences immediate feedback provided by robotic telescopes.
With such a large effort directed toward microlensing, there is a fair chance of detecting an earth-size planet in the coming years. This could be possible with an intricate network of wide-field telescopes or a space-based telescope. The detection limit of gravitational microlensing extends below 0.1 Earth-masses, which is very promising for finding the coveted earth-sized planets.
The GLIESE 581 Planets
The first extra solar, Earth-like planet was discovered by the Stephane Udry University of Geneva's Observatory in Switzerland in April 24th, 2007. This terrestrial planet is called GLIESE 581C and has a mass at least five to eight earth masses. There is another terrestrial planet in the GLIESE 581 system. The second planet GLIESE 581C lies in front of the theoretical “habitable zone” with an orbital period of 12.9 earth days while the third planet, GLIESE 581D, lies outside the habitable zone, having an orbital period of 83.6 earth days. Both these planets have very eccentric orbits which makes their orbital periods short compared to Venus, Earth and Mars in our solar system. These are the first two terrestrial planets discovered outside our solar system, which are earthlike in size and orbital distance from the parent star.
Scientists believe GLIESE 581c is too close to its star for water to exist on it, and GLIESE 581d may be too far away to support life, giving these two planets a “goldilocks” syndrome. The star in the GLIESE system is a red dwarf main sequence M star on the HR diagram. An M star has the ability to tidally lock its planets in the habitable zone. As a result of this tidal effect, one side of the planet would always face the star, and the other side would remain in darkness, giving rise to massive temperature differences. The tidal effect would never allow the sun-facing hemisphere to cool down, since the star’s rays would always be hitting the same side of the planet. This causes one side of the planet to be too hot to sustain water, which is probably the case for GLIESE 581c.
Scientists believe GLIESE 581c is too close to its star for water to exist on it, and GLIESE 581d may be too far away to support life, giving these two planets a “goldilocks” syndrome. The star in the GLIESE system is a red dwarf main sequence M star on the HR diagram. An M star has the ability to tidally lock its planets in the habitable zone. As a result of this tidal effect, one side of the planet would always face the star, and the other side would remain in darkness, giving rise to massive temperature differences. The tidal effect would never allow the sun-facing hemisphere to cool down, since the star’s rays would always be hitting the same side of the planet. This causes one side of the planet to be too hot to sustain water, which is probably the case for GLIESE 581c.
The Kepler Mission
In our search for more knowledge about whether Earth is the only terrestrial planet in the universe, NASA has launched a mission capable of finding Earth-sized extra solar planets. First formed in March 2001, the Kepler Mission’s objectives include exploring the structures of planetary systems by surveying large numbers of stars in the galaxy and determining these planets sizes and shapes. The Kepler spacecraft contains a space photometer, which will be able to observe over 100,000 stars simultaneously while in orbit. This will give us an idea of how many Earth-sized planets exist in the solar system and hopefully provide information as to whether any of these planets may be habitable.
The tentative launch date is March 6, 2009, and according to NASA, the duration of the mission will be around 3.5 to 9 years. There are high expectations for this mission, and if all goes according to plan, NASA expects to detect about 50 or more Earth-sized planets, as well as some 800 other planets slightly larger than Earth. The photometer will point at the constellation Cygnus, which is not obscured by either the Kuiper belt or any asteroid fields. It is NASA’s hope that this mission, unlike past missions, will answer our questions concerning the existence of other Earth-like planets in this vast universe.
The CoRoT Mission
The French “COnvection, ROtation and planetary Transits”-satellite (CoRoT), is currently using photometry—measuring the intensity of an object’s radiation, in this case at the optical range—to search for extra-solar planets. Starting in 2001 and launched in 2006, the mission hopes to monitor about 30,000-60,000 stars overall. Because of its high photometric accuracy, CoRoT is discovering Neptune- and Uranus-like planets and is attempting to detect large terrestrial planets, down to approximately 2 Earth radii. This year, CoRoT has picked up signals as small as 5 parts in ten thousand. If these signals are a planet’s “transit”—the event of the planet moving between its star and the CoRoT satellite—the planet’s radius would be 1.7 Earth radii. Although astronomers have yet to confirm the “planetary nature” of the signal, this discovery is providing the next step in Earth-sized planet detection.
In Conclusion...
Astronomers have approached the problem of finding extra solar Earth-sized planets in many different ways. They have moved past techniques better suited for Jovian planet detection—notably astrometry, interferometry and radial velocity—and have looked toward microlensing, a more promising method. Moving from ground-based to space-based investigation, the Kepler and CoRoT missions are propelling the work to find Earth-sized planets many light-years away. The GLIESE 581C and 581D planets are also paving the way for possible discovery of habitable terrestrial planets. With this advanced technology, astronomers are attempting to uncover more about the universe in which we live.
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