HOW EXOPLANETS ARE DETECTED?
Adapted from Sara Seager’s article “Is There Life Out There? The Search for Habitable Exoplanets”.
Researchers have developed a handful of techniques to spot planets outside our solar system. These techniques are often used in combination to confirm the initial discovery and learn more about the planet’s characteristics. Here is an account of the main methods.
Transits. When a planet crosses or ‘transits’ the face of its host star, the star’s light dims by a small but detectable amount (Figure 1). The probability that any planet’s transit will be visible is dictated by the relative sizes of the star and the planet. Planetary transits are only noticeable for planets whose orbits happen to be perfectly aligned from the astronomers’ line of vision. About 10% of planets with small orbits have such alignment, and the fraction decreases for planets with larger orbits. Using this method researchers can deduce the radius of a planet and its orbital period. Sometimes planet’s atmosphere can be studied if starlight filters through or reflects off it. This gives information on atmospheric composition, temperature and cloud formation.
Radial Velocity. The Radial Velocity method is currently the most effective method for locating extrasolar planets. Radial velocity is the motion of a star caused by the gravitational influence of its orbiting planets. Radial velocity measurements can detect only planets whose orbits tug the star towards and away from the observer through the increases (blueshifts) or decreases (redshifts) in the frequency of light the star emits. The exact orbit of an exoplanet is difficult to determine, so radial-velocity measurements let researchers deduce only the time a planet takes to orbit the star (it orbital period), how its orbit deviates from circular. Radial velocity is most sensitive for massive planets with short orbital periods. The main drawback is it cannot accurately determine the mass of a distant planet, but only provides an estimate of its minimum mass.
Scientists can track a star’s spectrum using highly sensitive spectrographs that detect periodic shifts of spectral lines towards the red and blue end of its spectrum. Periodic shifts occurring at fixed intervals of days, months, or even years, indicate that the host star’s back and forth or cyclic motion towards the Earth caused by a body such as a planet orbiting the star.
The Radial Velocity method is unlikely to find Earth-size planets that could harbor life. In fact, most of the planets detected by spectroscopy are “hot Jupiters.” Cooler planets that are further away from their host star are much harder to detect with spectroscopy as they produce less wobbles in their host star’s trajectory and take years to complete a revolution around their host star.
Direct Imaging: A direct image of an exoplanet system is a snapshot of the planets and disk around a central star. Scientists can estimate the orbit of a planet from a time series of images. Direct imaging can provide information about the size, temperature, clouds, atmospheric gases, surface properties, rotation rate, and likelihood of life on a planet from its photometry, colors, and spectra in the visible and infrared. The technique is appropriate for detecting massive planets which have orbits larger than that of the Neptune in our Solar system.
So far, direct imaging has led to the discovery of a handful of exoplanets, including the planets around the stars HR 8799 (Figure 3), Fomalhaut, and Beta Pictoris. The key to the success of direct imaging is the ability to suppress the host star’s overwhelming brightness, else these planets are completely lost in the glare of their host star. For example, if our solar system were viewed from a vantage point that is 70 light years away (average for a nearby star), Jupiter would appear roughly a billion times fainter than our Sun. This is equivalent to viewing a dime from a distance of 5 miles.
Sophisticated instruments or “oculars” are being designed for the largest ground-based telescopes that can (1) suppress the host star’s image and diffraction pattern, and (2) suppress the star’s scattered light from imperfections in the telescope. Studies have shown that direct imaging of exoplanets may become routine using ground-based observatories. Scientists have also proposed aligning a specially shaped occulting screen with the James Webb Space Telescope (JWST), to be launched in 2018, for a detailed spectroscopic characterization of planetary mass companions of various stars.
Astrometric Method. Using this method, astronomers look for the motion of the star (wobbles) about the common center of mass (both the star and the planet move around a common center of mass). Astrometric instruments can precisely measure the position of stars as compared to other stars around them, and are thus able to detect any movements in the star’s position due to the “wobbling” caused by an orbiting exoplanet. Planets in our solar system have a similar effect on the Sun, producing a to-and-fro motion that could be detected by an observer positioned several light years away (Figure 4).
Transiting Exoplanet Survey Satellite (TESS)
NASA’s TESS mission (Transiting Exoplanet Survey Satellite), scheduled for launch in 2017, will survey nearby stars for transiting exoplanets. Transiting exoplanets are those that pass in front of their parent star as seen from the telescope, and this is the same technique NASA’s Kepler mission used to discover more than 3,500 exoplanet candidates. TESS will carry four identical specialized wide-field CCD cameras, each covering 24 degrees x 24 degrees on the sky with a 100 mm aperture. In a two-year all-sky survey of the solar neighborhood, TESS will cover 400 times as much sky as did Kepler. In the process, TESS will examine more than a half million bright, nearby stars, and will likely find thousands of exoplanets with orbital periods (i.e. “years”) up to about 50 days. TESS will be capable of finding Earth-size and super Earth-size exoplanets (up to 1.75 times Earth’s size) transiting M stars, stars which are significantly smaller, cooler, and more common than our Sun. TESS is projected to find hundreds of super Earths with a handful of those in an M star’s habitable zone. Extensive follow-up observations by ground-based observatories in the United States and internationally will then be used to measure the planet mass to confirm the exoplanets as being rocky.
James Webb Space Telescope (JWST)
NASA’s James Webb Space Telescope (JWST), scheduled to launch in 2018, will be capable of studying the atmospheres of a subset of the TESS rocky exoplanets in visible, near infrared, and infrared light. The technique JWST will use is called transit spectroscopy. As a transiting exoplanet passes in front of its host star, we can observe the exoplanet’s atmosphere as it is backlit by the star. Additional atmospheric observations can be made by observing as the exoplanet disappears and reappears from behind the star. In these observations the exoplanets and their stars are not spatially separated on the sky but are instead observed in the combined light of the planet-star system. We anticipate TESS will find dozens of super Earths suitable for atmosphere observations by JWST, including several that could potentially be habitable. The chance for life detection with the TESS-JWST combination — albeit small — is a possibility if life turns out to be ubiquitous.