430! That’s the number of exoplanets we discovered until the beginning of 2010. How did we do it? With the help of state-of-the-art spectrographs, interferometers, earth based or space telescopes, top-notch software and of course long hours of carefully examining the skies hopping to find planetary prospects for life’s development.

First of all. it’s important to state that the planets were not observed directly; we didn’t yet invent a telescope that could delve its way with enough clarity and magnification capabilities through the tens of light years of distance away, so that it could offer us a compelling and clear picture of an extrasolar planet. At least not yet.

All planets thus detected were observed by indirect means. By taking advantage of the prospective extra-solar planet’s effects on the star which it orbits, effects that manifest themselves by affecting the movement, brightness and characteristics of the light spectrum of that star, scientists have thus far discovered a myriad of exoplanets, even though the majority of them are massive gas giants like Jupiter, Saturn, Uranus and Neptune. The most known four general methods used to detect exoplanets are: (1) The Doppler Shift (or Radial Velocity) Method, (2) The Transit Observation Method, (3) The Astrometric Measurement Method and (4) The Gravitational Microlensing Method. Beside these we will talk about (5) The Direct Detection Method, (6) The Nulling Interferometry Method and (7) The Polarimetry Method. Let’s begin!

        (1) The Doppler Shift (or Radial Velocity) Method is the first and the most widely used method, put into service in general for the detection of massive gaseous extrasolar planets. How does it work? Imagine a star with a orbiting planet circling it. Although the planet is the one that seems to perform the circular movement around the massive star, in reality the planet orbits a so called “center of mass of the planetary system”, that is, a point at which the two giant bodies balance each other. Because the planet is the one with a much smaller mass, it will of course have a much larger radius while orbiting this center of mass, but none the less, the star too will orbit around this center, although with a much smaller radius, and this is exactly what the scientists are looking at. The Radial Velocity Method, or Doppler Shift Method, takes advantage of the fact that when the star is wobbling back and forth, from out point of view, it also changes the light spectrum we observe here on Earth. Thus, if the star moves slightly away from an observer on Earth, we will see the emitted light from the star as shifted towards longer wavelengths (shifted towards red), and towards shorter wavelengths (shifted towards blue) as it slightly approaches Earth.

This back and forth movement of the star, analyzed through the shift in the spectrum, gives the orbit and mass of the planet. The bigger the planet, the bigger the wobbling therefore the greater the shift in color, consequently that’s why the vast amount of planets found with this method are gas giants.

A proper use for this method so that we could find even smaller and distant planets, that is, smaller than giants like Jupiter and Saturn, is to target it towards smaller stars like red dwarfs, namely, stars that are smaller than half the size of the own Sun. They are smaller and colder than other stars because they burn their fuel slowly, so they shine less light than hot stars. Thus, because of their small mass, the effects exerted by a prospective orbiting planet would be observed much easier by an observer on Earth.

Although it is a giant planet, Gliese 876b is the first discovered exoplanet orbiting a red dwarf, and found by the Hubble Space Telescope to be 1.89 to 2.4 times more massive than Jupiter, our solar system’s largest orbiting planet. It has two more brothers orbiting the same Gliese 876 star, located in the constellation of Aquarius, 15 light years away from Earth, namely, Gliese 876c, and Gliese 876d which is estimated to have mass a 5.88 times greater than that of Earth and is thought to be similar in characteristics to out own planet. Gliese 876d takes less than two days to complete an orbit, at a distance of only 1/50 of that between Earth and our own star, and is the innermost planet it its planetary system. The interesting news about this planet is that it’s located within the habitable zone, that is to say it is located in the region distanced properly away from the hosting star, so that liquid water can be maintained on its surface.

        (2) The Transit Observation Method uses the apparent dimming in brightness of a star’s light as seen by an observer on Earth, to predict the actual existence of the prospective planet, which has its orbital plane close to the line of sight of the observer, when it passes in front of the star, thus reducing the amount of light that reaches Earth.

Courtesy NASA/JPL-Caltech

The first exoplanet discovered by this method is HD 209458b located 150 light-years away in the constellation of Pegasus . The planet orbits its sun in 3.5 days, has an estimated surface temperature of about 1,000°C (~ 1,800°F), a mass 220 times that of Earth and the volume 146% greater than that of Jupiter. With the help of NASA’s Hubble Space Telescope and Spitzer Infrared Space Telescope astronomers revealed that HD 209458b has an atmosphere containing hydrogen, carbon, oxygen and sodium.

The most successful search for exoplanets with the transit observation method is the Wide Angle Search for Planets (SuperWASP) project. Their two observatories, SuperWASP-North located in La Palma at the Isaac Newton Group of Telescopes, and SuperWASP-South at the South African Astronomical Observatory. Why are these installations so special? Their 16 wide-angle cameras, that is 8 for each observatory, with state-of-the-art CCD detectors. The computers continuously scan the sky, monitoring millions of stars in the hope of detecting transit events. So far the survey managed to discover 30 new exoplanets since its start in 2004. Another project is the WFCAM Transit Survey that uses the wide-field camera at the UK Infra-Red Telescope (UKIRT) in Hawaii, to search for exoplanets orbiting red dwarf stars.

As for dedicated space missions using this method for the detection of exoplanets we have ESA’s COROT mission and NASA’s KEPPLER mission. COROT (COnvection ROtation and planetary Transits) is a space mission led in conjunction by the European Space Agency (ESA) and the French Space Agency (CNES). It’s main purpose? The search for extrasolar planets with small orbital periods (big planets) and to conduct astroseismology studies by measuring the oscillations in stars. With its 27 centimeter telescope and state-of-the-art CCD detectors it managed to discover the smallest planet known until 2010: the COROT 7b, which has a diameter 1.7 times bigger than that of the Earth, and a mass 4.8 times greater than our planet’s. As for the KEPPLER mission, the objectives are even more ambitious; the NASA scientists expect to find about 50 planets with the size of Earth. The space telescope is equipped with a 1.4 meter diameter primary mirror, and 42 CCDs with 2200×1024 pixels giving it a total 95 mega pixels of clean-cut pure imaging.

KEPPLER - Courtesy NASA/JPL-Caltech

        (3) The Astrometric Measurement Method, just like the RV (Radial Velocity) Method, uses the star’s motion caused by the orbiting planet’s tug, but this time instead of looking at the apparent shift in the spectrum of light emitted by it, the scientists are looking directly at the tiny displacements of light caused by its to-and-fro motion (wobble). Due to the very demanding hardware installations required on the ground based observatories, because atmospheric interference limits the accuracy of the measurements, there are few astrometric observatories that search for extrasolar planets. The most known are the W.M.Keck observatories in Hawaii, which have the unprecendented accuracy of 20 micro arcseconds, this being equivalent to an observer on Earth distinguishing a golf ball placed on the Moon. Keck’s mission? To survey with the astrometric method hundreds of stars, in hope of finding planets with masses as small as our solar system’s gas planet, Uranus.

Although there are little prospects to find earth-like extrasolar planets with earth based observatories using this method, rest assured because scientists at the European Space Agency will launch in 2011 the Gaia astrometry space mission with the goal of uncovering thousands of planets within 650 light years away using both transit and astrometric methods. For this mission, several UK university groups are joined in the Gaia Data Processing and Analysis Consortium (DPAC) to study the data that Gaia will gather from millions of stars. Nonetheless, NASA’s Space Interferometry Mission (SIM), that has been canceled several times, its due to be launched in 2015 and will be able to make angle measurements of single stars “as accurate as 1 micro arcsecond – the width of a human hair at a distance of 500 miles”. Additionally, PRIMA, a highly accurate astrometric instrument, will operate on one of the four telescopes of the ESO’s VLT array and will be able to detect more earth-like exoplanets with the astrometric method. The VLT array is a group of four telescopes, Antu, Kueyen, Melipal and Yepun, located on Cerro Paranal, that is operated by the European Southern Observatory (ESO).

VLT Array - Courtesy ESO

        (4) The Gravitational Microlensing Method. Gravitational lensing is the phenomenon by which the light that comes from a distant and bright object, because it is bent by a massive galaxy or a black hole, appears to be coming from multiple directions, from the point of view of an observer that has that massive galaxy between him and the bright object. You virtually can see double images of the same object, or explained inversely, you can see the same object in several positions at the same time. Additionally, the phenomenon manifests itself through ringlike distortions of matter, also called Einstein’s rings sometimes, because the famous physicist predicted their existence by deriving their mathematical formulas from his general theory of relativity.

Courtesy NASA/JPL-Caltech

But, the same laws of physics that drive the phenomena above, may also cause the brightening of a star when a foreground microlensing planetary system, close to the line of sight that leads to the observer on Earth, finds itself in front of it. When the orbiting planet of the foreground distanced sun happens to be in the right position, it will produce a sudden increase or decrease in the brightness of its sun, of course as it will be seen by an observer on Earth.

Having these amplifying characteristics, the microlensing effect can be used to our own advantage by making it a tool for discovering small planets like our own, because it is more sensitive to low-mass planets than other methods.

The Optical Gravitational Lensing Experiment (OGLE), Microlensing Observations in Astrophysics (MOA) and the PLANET are some examples of the constituents of a worldwide network of programmes that search for extrasolar planets using the microlensing method.

GRAVITATIONAL MICROLENSING / Courtesy - Department of Astrophysics, UNSW, Sydney, AUSTRALIA

GRAVITATIONAL MICROLENSING

        (5) The Direct Detection Method. That is, directly looking at the visible and infrared light emitted by giant planets that circle their stars in wide orbits. It may seem a real challenge, and it really is, but current advances in CCD technology make it possible to detect these planets, not to long ago undetectable with this method. Using this technique the Fomalhaut b exoplanet, 25 light years away from Earth, was detected using the Hubble Space Telescope; the operation used a coronagraph, that is, a disk used to block a significant part of the light emitted by the star it is aimed at.

Fomalhaut system as seen by the Hubble Space Telescope - Fomalhaut b orbits its sun in 872 years

        (6) The Nulling Interferometry Method. This method uses the light waves received at several telescopes and adds them with the interferometry technique, so that the waves cancel each other out in such a way that the only thing that remains in the final picture is the hoped exoplanet.

Nulling Interferometry Method / Courtesy - ESA

        (7) The Polarimetry Method. This method is based on the phenomenon of light, emitted by the hosting star, changing its polarization as it bounces with the atmosphere molecules on the planet that orbits it. Checking for discrepancies in the light polarization from the areas that make the star, scientists can deduce the existence and even study the properties of the planet’s atmosphere.

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References:
- Institute of Physics – EXOPLANETS -> The search for planets beyond our solar system
- WIKIPEDIA – Extrasolar Planet