Astronomy - Exoplanets

Exoplanets

This space we declare to be infinite... In it are an infinity of worlds of the same kind as our own.

Giordano Bruno (1584)

In the sixteenth century the Italian philosopher Giordano Bruno, an early supporter of the theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.

An exoplanet (extrasolar planet) is a planet located outside the Solar System.

The first evidence of an exoplanet was noted as early as 1917 but was not recognized as such.

The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet , “a hot Jupiter” 50 light-years away was found in a four-day orbit around the star 51 Pegasi.

                          Michael Mayor                            Didier Queloz

First Exoplanet Orbiting a Main-Sequence Star

As of 2 June 2018, there are 3,786 confirmed planets in 2,834 systems, with 629 systems having more than one planet.

There are currently about fifty known exoplanets whose diameters range from Mars-sized to several times the Earth's and which also reside within their stars' habitable zone – the orbital distance within which their surface temperatures permit liquid water. These exoplanets are currently our best candidates for hosting life.

 

In the first observation of its kind, the Hubble Space Telescope has found helium in the atmosphere of a Jupiter-class exoplanet 200 lightyears from Earth.

Another team of researchers, using Europe’s Very Large Telescope in Chile, has found an exoplanet, WASP-96b, with a cloud-free atmosphere, allowing them to detect sodium in levels similar to abundances on Earth.

Main Methods of Detecting Exoplanets

Direct Imaging                         

Transit                                     

Radial Velocity                        

Microlensing                           

Direct Imaging

Any planet is an extremely faint light source compared to its parent star.

For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it.

In addition to the intrinsic difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out.

Planets orbiting far enough from stars to be resolved reflect very little starlight, so planets are detected through their thermal emission instead.

it is easier to obtain images when the star system is relatively near to the Sun, and when the planet is especially large (considerably larger than Jupiter), widely separated from its parent star, and hot so that it emits intense infrared radiation.

For those reasons, very few of the extrasolar planets reported as of April 2014 have been observed directly, with even fewer being resolved from their host star.

Direct imaging of exoplanets that is, actual pictures will play an increasingly larger role.

Transit

The vast majority of exoplanets have been found by searching for shadows: the incredibly tiny dip in the light from a star when a planet crosses its face. Astronomers call this crossing a “transit.”

The size of the dip in starlight reveals how big the transiting planet is.

NASA’s Kepler space telescope, launched in 2009, has found nearly 2,700 confirmed exoplanets this way.

The main advantage of the transit method is that the size of the planet can be determined from the light curve.

The transit method also makes it possible to study the atmosphere of the transiting planet. When the planet transits the star, light from the star passes through the upper atmosphere of the planet.

By studying the high-resolution stellar spectrum carefully, one can detect elements present in the planet's atmosphere.

This method has two major disadvantages. First, planetary transits are observable only when the planet's orbit happens to be perfectly aligned from the astronomers' vantage point.

The second disadvantage of this method is a high rate of false detections. A 2012 study found that the rate of false positives for transits observed by the Kepler mission could be as high as 40% in single-planet systems.

Radial Velocity

A star with a planet will move in its own small orbit in response to the planet's gravity.

This leads to variations in the speed with which the star moves toward or away from Earth.

The radial velocity can be deduced from the displacement in the parent star's spectral lines due to the Doppler effect. The radial-velocity method measures these variations in order to confirm the presence of the planet.

The size of the wobble reveals the “weight,” or mass, of the planet.

Until around 2012, the radial-velocity method was by far the most productive technique used by planet hunters (after 2012, the transit method from the Kepler spacecraft overtook it in number).

The radial velocity signal is distance independent, but requires high signal-to-noise ratio spectra to achieve high precision, and so is generally used only for relatively nearby stars, out to about 160 light-years from Earth, to find lower-mass planets.

 

Gravitational Microlensing

Gravitational microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. This effect occurs only when the two stars are almost exactly aligned.

Lensing events are brief, lasting for weeks or days, as the two stars and Earth are all moving relative to each other. More than a thousand such events have been observed over the past ten years.

 

 

If the foreground lensing star has a planet, then that planet's own gravitational field can make a detectable contribution to the lensing effect. Since that requires a highly improbable alignment, a very large number of distant stars must be continuously monitored in order to detect planetary microlensing.

This method is most fruitful for planets between Earth and the center of the galaxy, as the galactic center provides a large number of background stars.

The main advantages of the gravitational microlensing method are that it can detect low-mass planets (in principle down to Mars mass); it can detect planets in wide orbits comparable to Saturn and Uranus, which have orbital periods too long for the radial velocity or transit methods; and it can detect planets around very distant stars.

A notable disadvantage of the method is that the lensing cannot be repeated, because the chance alignment never occurs again.

In addition, the only physical characteristic that can be determined by microlensing is the mass of the planet, within loose constraints.

Observations are usually performed using networks of robotic telescopes. In addition to the European Research Council-funded OGLE, the Microlensing Observations in Astrophysics group is working to perfect this approach.

Timing

The transit timing variation method considers whether transits occur with strict periodicity, or if there is a variation. When multiple transiting planets are detected, they can often be confirmed with the transit timing variation method. This is useful in planetary systems far from the Sun, where radial velocity methods cannot detect them due to the low signal-to-noise ratio.

If a planet has been detected by the transit method, then variations in the timing of the transit provide an extremely sensitive method of detecting additional non-transiting planets in the system with masses comparable to Earth’s. It is easier to detect transit-timing variations if planets have relatively close orbits.

The main drawback of the transit timing method is that usually not much can be learned about the planet itself.

Transit timing variation can help to determine the maximum mass of a planet.

The next generation of space telescopes is upon us. First up is the launch of TESS, the Transiting Exoplanet Survey Satellite in April. This extraordinary instrument will take a nearly full-sky survey of the closer, brighter stars to look for transiting planets.

Kepler, the past master of transits, will be passing the torch of discovery to TESS.

TESS, in turn, will reveal the best candidates for a closer look with the James Webb Space Telescope, currently schedule to launch in 2021.