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Extrasolar Missions: Finding New Planets

The search for extrasolar planets is one of the most interesting fields in astronomy right now. In 10 years astronomers have found about 150 extrasolar planets, and the rate is accelerating for each year. Along with the acceleration astronomers are attempting to narrow down the size of the planets to match the size of extrasolar terrestrial planets, that is, Earth-sized planets. Terrestrial planets are believed to be most likely to harbour life.
Currently observatories such as the Keck telescopes (ground-based) and the Spitzer telescope (space-borne) help astronomers in their search for extrasolar planets. Though Earthsized planets are beyond the capabilities of current instruments. However, NASA and ESA, (the world's two leading space agencies) are planning to put observatories in space within the coming 10-15 years. Names of missions, like SIM (Space Interferometer Mission), TPF (Terrestrial Planet Finder), Kepler and Darwin have been circulating in the media. Below you will find information about these missions.


ESA's (European Space Agency) Darwin mission (launch: around year 2015) will consist of four "free-flying spacecraft" that will search for Earth-like extrasolar planets around other stars. Three of the spacecraft will have accurate light collectors of 3-4 meters diameter. These three spacecraft will redirect the light to one fourth and central spacecraft.
The four spacecraft will be able to combine their mirror-size to provide an extremely high-resolution imagery. When combined the telescopes will be able to work as one single telescope with a diameter of hundreds of meters. The four telescopes will need to stay in a very accurate position relative to each other, with millimetre marginal.
The wavelength at which astronomers are interested in studying the planets are absorbed by Earth's atmosphere, thus the decision to put the observatories in space. Because stars are in some cases billions of times brighter than the planets in visible wavelengths of light, the Darwin mission will search for these planets in the infrared part of the spectrum. This will reduce the contrast from billions of times more light from the star to millions of times more. This will make the detection of extrasolar planets easier. Another advantage with observing at infrared wavelengths is that life on Earth is most easily detected through infrared. The Darwin observatories will be able to detect the same gases present in Earth's atmosphere (oxygen, carbon dioxide and methane for example) in other extrasolar planets.
The Darwin mission will use a technique called nulling interferometry, which means that advanced technique will use delayers in the spacecraft to cancel out the light of the star while the light from the planets remains.

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Keck Interferometer

The twin Keck telescopes located on top of Mauna Kea, Hawaii have the world's largest mirrors. Each of the telescopes have a mirror with a diameter of about 10 metres. This large mirror is in turn constructed of many smaller mirrors. The Keck telescopes are a part of NASA's Origins programme, which also includes the TPF and the SIM missions. The Origins programme aims, in part to find extrasolar planets.
The Keck telescopes are located 85 meters apart from each other, which gives them an aperture of an 85 meter wide telescope when combined. This in turn gives the telescopes a resolution of 5 milliarcseconds (mas) at 2.2 microns, which makes it a suitable aid in the search for extrasolar planets. Read more here: Keck Interferometer Programme.

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The Kepler mission is named after the famous astronomer Johannes Kepler (1571 - 1630). Johannes Kepler was a celestial mechanicer who contributed with the three laws of Kepler, which describe the motions of celestial bodies.
The Kepler mission is due to launch in June, 2008 and is "NASA's first mission capable of finding Earth-size and smaller planets around other stars." The mission will search for terrestrial planets in the sun's vicinity in the Milky Way galaxy. The planets which it will study will be located in or near a star's habitable zone, which is a distance range from a star where liquid water could exist. The transit method will be used by Kepler to detect these extrasolar terrestrial planets, which is a method that studies the variation of brightness of a particular star when a large object, such as a planet passes infront of the star. When the transit occurrs, astronomers can detect the mass and distance of the planet. The Kepler spacecraft will continuously and simultaneously monitor the brightness of 100 000 main-sequence stars during the 4 year lifetime of the mission.
Kepler mission's scientific objectives are according to its web site:

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:

  1. Determine how many terrestrial and larger planets there are in or near the habitable zone of a wide variety of spectral types of stars;
  2. Determine the range of sizes and shapes of the orbits of these planets;
  3. Estimate the how many planets there are in multiple-star systems;
  4. Determine the range of orbit size, brightness, size, mass and density of short-period giant planets;
  5. Identify additional members of each discovered planetary system using other techniques; and
  6. Determine the properties of those stars that harbor planetary systems.

The Kepler Mission is designed to test the hypotheses that:

  1. Most stars like our Sun have terrestrial planets in or near the habitable zone;
  2. On an average two Earth-size planets form in the region between 0.5 and 1.5 AU, based on our Solar System and the accretion model of Wetherill (1996).

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SIM - Space Interferometer Mission

The SIM mission, which is scheduled to launch in 2011, will be put in an Earth-trailing solar orbit, which means that it will follow the path of Earth's orbit.
The mission is designed not only to search for Earth-sized habitable planets around the closest 100 - 250 stars, but it will also study the distances and masses of stars (which also includes binary star systems) located further away.
The telescope will be using optical interferometry. Interferometry is the process used to combine the signal from two or more telescopes to produce a sharper image than each telescope could achieve separately. The accuracy of the instruments will be unprecedented: the resolving power is 4 microseconds. One full circle is divided into 360 degrees, each degree is divided into 60 minutes. A minute is divided into 60 seconds. Therefore a full circle consists of 1,296,000 arcseconds. A microsecond is one millionth of an arcsecond. Needless to say, this resolving power will be enormous compared to the instruments that exist today (1000 times more poweful). To achieve this accuracy, the observatory needs to be placed outside the atmosphere of Earth, which distorts the light from the stars.
The search for habitable planets around the closest 100-250 stars will be using the full resolution of the spacecraft. However a few thousand stars will be studied at a lower resolution, in order to find Neptune-sized planets.
The SIM mission is scheduled to last for about 5 years, and will be located at a distance of 95 million kilometres at the end of those 5 years.

As far as nearby extrasolar planetary systems are considered SIM will (according to the whitepaper given to NASA by the JPL, available here):

  1. Exhaustively search the nearest ~75 stars for planets more massive than ~2 MEarth.
  2. Survey several thousand stars for Neptune-size and larger planets in orbits out to 10 AU.
  3. Enumerate the planets in multi-planet systems.
  4. Determine the orbits, including eccentricity and inclination.
  5. Determine unambiguously masses of planets, perhaps the most fundamental property governing the evolution and fate of a planet.

It should be noted that the number of stars that will be exhaustively studied has varied from 75 - 250.

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The Spitzer Space Telescope studies the universe through infrared wavelengths of the spectrum, which isn't limited to studying the search for extrasolar planets. It is considered as a cousin to the Hubble Space Telescope. The mission has been active in space since 2003 and contributes to the search for extrasolar planets by searching for dust discs around newly born stars where planets could be forming. The Spitzer telescope has also been used to directly view to extrasolar planets called HD 209458b and TrES-1, though the discovery was made using other methods first. It should be noted that the Spitzer telescope wasn't designed to search for these extrasolar planets, it's a bonus. Spitzer is ideally suited for studying extrasolar planets known to transit their stars.

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TPF - Terrestrial Planet Finder

The Terrestrial Planet Finder mission, which is scheduled for launch in 2011 is similar in design to the ESA's Darwin project. It too will be using a technique called nulling interferometry which will veil the light from the star and bring forth the light of the planets. The observatory will be capable of taking images 100 times more detailed than the Hubble Space Telescope and is aimed to find Earth-sized planets in the sun's vicinity. Instruments on the observatory will be able to reveal the relative amounts of gases like carbon dioxide, water vapor, ozone and methane. All these substances would tell if the planet found is capable of finding life.
According to NASA's TPF website the TPF will seek anwers to these questions:

  1. Are there Earth-like planets in the "habitable zones" around their parent stars where the surface temperature is capable of supporting liquid water over a range of surface pressures?
  2. What are the compositions of the atmospheres of terrestrial planets orbiting nearby stars? Is water, carbon monoxide, or carbon dioxide present?
  3. Are there atmospheric components or conditions attributable to primitive life, such as ozone or molecular oxygen, seen in the Earth's atmosphere?
  4. How do planets form out of disks of solid and gaseous material around young stars?

It is likely that the TPF mission will use the same kind of configuration that the Darwin project project will use, in the sense that 3-4 observatories will be collecting light and transmitting it to one central observatory. The diamter of these observatories will be 3.5 meters.

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Keck Interferometer
SIM - Space Interferometer Mission
Spitzer Telescope
TPF - Terrestrial Planet Finder


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