Extrasolar Planets

An Extrasolar planet, or exoplanet, is a planet outside the Solar System. First exoplanet was confirmed indirectly at G-type star 51 Pegasi. So far, about 500 planets were confirmed through the astronomical observations. In this chapter, we study the detection method, the structure and the evolution of exoplanets.

Detection Methods

Exoplanets are an extremely fainter than those of central stars. Glares from the stars obscure the faint light sources. For the reason, only a very few extrasolar planets have been observed directly. The following are the indirect method that have been applied for the detection of known exoplanets.

Doppler technique Astrometric Technique Transit Technique Gravitational microlensing Circumstellar disk Direct detection

Doppler method A planet exerts a small gravitational pull on its parent star, causing the star to wobble. The motion amplitude depends on the orbital distance and the mass of the planet. The motion of star is detectable with the Doppler Effect. The light coming from a star moving toward the Earth will be Doppler shifted to bluer (shorter) wavelengths, whereas a star moving back from the Earth will emit light shifted to redder (longer) wavelengths.

By making precise measurements of the frequency of absorption lines in the star's spectrum, it is possible to see this alternate blue- and red-shift effect. The Doppler spectroscopy allows one to estimate the distance between planet and central star, and place a lower limit on the mass of the planet. The first exoplanet was found at 51 Pegasi in 1995 by using the Doppler technique. The Doppler spectroscopy technique has been the most successful so far in finding extrasolar planets.

 orbital period = 4.2 days  semi-major axis = 0.05 AU  evaluated mass = 0.5 Jupiter Masses Constellation: Pegasus 5.5 mag star

The wobbling effect is very small. In our Solar System, Jupiter exerts the strongest force on the Sun with a radial velocity of 12 m/s. On the other hand, Earth have the effect only 10 cm/s, over a period of a year. For the reason, more massive planets and the closer distance from the central stars are selectively found. The Dopple method is most effective for edge-on view, but not for face-on view.

Planetary Systems, Ollivier et al., Springer

Astrometry method 'Astrometry' is a measurement of stellar position. With the help of astrometry, astronomers study the precise, periodic wobble that a planet induces in the position in the sky of its parent star. Unlike the Doppler method, astrometry method works best when the orbit of the planet around the star is perpendicular to the viewer. Also, a planet that orbits far away from its star will be more easily detected as it will cause a greater shift in the position of the star. However, planets at greater distances from the star require longer time. In total it could take many years, perhaps decades.

Motion of the Sun as a function of time (in years) on the plane of the sky as it would appear from a location 10 parsecs away and perpendicular to the plane of the ecliptic. This movement is mainly dominated by the giant planets (Jupiter, Saturn, Uranus, and Neptune).

Transit Method This method uses the fact that when a smaller object passes in front of a host star, the star appears to fade in luminosity. Even if the reduction is very small (typically between 0.01% and 1%) astronomers can detect it through the photometry (measurement of the brightness of stars). The event could be regarded as an eclipse or an "occultation". Transit by VenusSolar Eclipse

The photometric transit method has an disadvantage in that the star which is being studied needs to be edge-on. This method could work on great distances. An advantage this method has is that during the occultation, the composition of the planet's atmosphere could be detected. The study of such an occultation would produce a light curve, which would show how much a star had faded due to the passage of the planet. If the curve is precise enough, it could even reveal the presence of moons around the planet and astronomers would know immediately if the planet is in the habitable zone.

Diagram illustrating the principles of a planetary transit in front of a star, and the associated photometric light-curve

The transit will be visible only if the line of sight intercepts the cylinder, constructed on the orbit, of radius a p and height 2r ∗. For a circular orbit, the following relationship may be derived:

Example: Let us derive the probability of the transit of the Earth seen from the other planetary systems. 2r * = 1.4x10 6 km a P = 1 AU= 1.5 x 10 8 km P ~ = 1%

A transit of HD b observed (left) from the ground and from space with the HST

Gravitational Microlensing Method According to general relativity, mass "warps" space–time to create gravitational fields and therefore bend light as a result. Microlensing is a phenomenon that occurs when an object with enough mass passes between us and a background star. If a planet and a star would happen to pass in front of a background star, the background star's luminosity would appear to increase (because light is bent by the planetary- system's gravity). This is a very promising and new method, though the chance is low that a planet-star system would pass between us and a background star. For this reason, it is more efficient to study a background with many stars, for example a view towards the galactic centre would provide a significant amount of stars.

Detection via microlensing OGLE-2003-BLG-235

Circumstellar Disk Disks particles surround many stars (debris disks or circumstellar disk). The dust can be detected because the dust particles have a large total surface area. Dust disks have now been found around more than 15% of nearby sun-like stars. The dust is believed to be generated by collisions among asteroids or sublimating of ice from comets. Radiation pressure from the star drags the dust particles into the stars (Poynting-Robertson effect). Therefore, the detection of dust indicates continual replenishment by new collisions, and provides strong indirect evidence of the presence of small bodies around the parent star.

Stationary Moving train Moving train loses the momentum by the collision with droplet Poynting-Robertson Effect

Kalas et al. 2005

Advantages of the Doppler Method –Most successful method –About 85% of known exoplanets are detected by the technique –The Dopplet method is sensitive to massive planets around relatively nearby stars Advantages of Transits –Transits offer the only way we currently have to make a direct measurement of the radii of exoplanets –Gives an estimate of the density –Densities are important clues to the composition of the exoplanet (gas giant, ice giant, rocky planet, etc.) –The only way we have to probe the atmospheres of exoplanets –The latest application of the Transit Method from space holds out the possibility of detecting Earth-mass planets. e.g. the European COROT satellite and the US KEPLER mission.

Advantages of Microlensing –Microlensing is superbly sensitive to planetary systems like our own Solar System –It is not biased towards finding close-in Jupiters like the RV or Transit methods –In principle, Microlensing may be the only way we currently have that could detect Earth-mass planets from the ground. –This is much cheaper than expensive space missions, and can be done by networks of small amateur and professional telescopes.

Diversity of Exoplanets

As of November 11, 2010, all of these techniques have found 496 planets with hundreds more planet candidates awaiting to be confirmed by more detailed investigations. Most are Jupiter-sized or larger (up to 13 times Jupiter's mass), with some recent detections getting into the Neptune range, and a couple of tantalizing "Super Earths" down to the several Earth mass range. It is now known that a substantial fraction of stars have planetary systems, including at least around 10% of sun-like stars. It follows that billions of exoplanets must exist in our own galaxy alone.

None of the planetary systems found so far resembles our Solar System. The biggest surprises are findings of many Jupiter-sized planets very close to their parent stars Some, called "Hot Jupiters", are on orbits smaller than that of Mercury, and have periods less than 10 days! What is going on? This is a subject of much current research. The discovery of extrasolar planets has great interest in the possibility of extraterrestrial life. As of September 2010, Gliese 581 g, fourth planet of the red dwarf star Gliese 581, appeared to be the best known example of a possibly terrestrial exoplanet orbiting within the habitable zone that surrounds its star.

Gliese 581g: orbital period=37 day >3.1 Earth mass

It is estimated that the average global equilibrium temperatureof Gliese 581 g ranges from 209 to 228 K (-64 to -45°C) for Bond albedos from 0.5 to 0.3. Adding an Earth-like greenhouse effect yields an average surface temperature in the range of 236 to 261 K (-37 to −12 °C). A factor that could potentially give Gliese 581 g a greenhouse effect greater than Earth's is the possibility the more massive planet also has a more massive atmosphere.

From: Review by G. Marcy Ringberg 2004 Two-planet system: Gliese 876

From: Review by G. Marcy Ringberg % of detected planetary systems are known to be multiple mean motion resonances Multiple Planetary Systems

Hot Jupiters