The discovery of the first planet outside of our own solar system was announced on 9 January 1992, by radio astronomers Aleksander Wolszczan and Dale Frail. The planet orbits a pulsar and was detected from variations in the pulsar’s period, due to the planet‘s gravitational influence.
Since then, thousands of extrasolar planets (or exoplanets) have been detected around stars through various methods – details of which are given below.
Around a fifth of all Sun-like stars are believed to host a planet roughly the size of the Earth within the habitable ‘Goldilocks’ zone – the distance from the star where it is neither too hot, nor too cold for a planet to support liquid water. This suggests there are at least 11 billion potentially habitable Earth-sized planets, and over 100 billion planets in total, in our Milky Way galaxy alone.
In August 2016, it was announced that a rocky planet had been discovered orbiting our nearest neighbouring star, Proxima Centauri, just 4.24 light-years away. Although Proxima Centuri is far fainter than our own Sun, the planet lies closer to its star than Earth does to our Sun, putting the planet within what is considered to the habitable zone around the star.
Radial Velocity Measurement and Similar Methods
The gravitational pull of a large planet will cause its star to orbit the combined centre of mass, or barycentre, of the system. As stars are generally much more massive than the planets that orbit them, this barycentre is usually well within the radius of the star, such that this orbital motion may appear as a very slight wobble in the star’s position. If the star’s radial velocity, towards and away from the Earth, is sufficiently large, it can be detected from a shift in the star’s spectral lines due to the Doppler effect.
The eccentricity of the planet’s orbit can be determined from the radial velocity method, as well as a minimum estimate of the planet’s mass. The true mass of the planet can also be determined when combined with the transit photometry method described below.
One of the advantages of this method is that it is dependent on the star’s radial velocity rather than its distance, although it requires a good signal-to-noise ratio in the star’s spectrum.
Relativistic beaming is a related method that detects changes in the density of photons from the star, and therefore its apparent brightness, due to its motion. Although the light variations are very small, this method doesn’t require a well-resolved spectrum.
Ellipsoidal variations in a star’s shape, due to tidal influences from a large planet, have also been used as a method of detection, from changes in a star’s apparent brightness.
Astrometry measurements can also be used to directly detect changes in a star’s position in the sky, indicating that a planet is causing it to orbit the barycentre of the combined system. This is the oldest method used to search for extrasolar planets, dating back to the 19th century, although none of the previously claimed discoveries have survived scrutiny. It is likely that the Gaia space observatory, launched in 2013 will use this method to detect thousands of Jupiter-sized exoplanets.
Prior to 2012, the radial velocity method was the most productive means of detection. However, the transit method has since taken over, due to the launch of the Kepler space observatory.
This method exploits the slight dimming in the light from a star as a planet passes in front of it.
Transit photometry can be used to estimate the planet’s size and possibly even measure the composition of the planet’s atmosphere, through spectral analysis.
Unfortunately, this method is highly dependent on the alignment of the planet’s orbital plane relative to the line of sight from the Earth, as well as the position of the planet in its orbit when a survey of the star is taken. For a planet orbiting a star the size of the Sun with the same orbital radius as the Earth, the probability of a random alignment producing a transit is less than 0.5 per cent.
Transit timing can also be used to detect the presence of additional non-transiting planets that perturb the orbit of the transiting planet.
Reflection or Emission modulations
Planets close to their host stars can sometimes be detected from changes in their apparent brightness, as they exhibit phases like those of the Moon, or Venus. Although planets cannot be resolved from the light of the star by this method, it does have the advantage of being able to detect planets irrespective of their orbital inclination relative to the Earth (as long as they are not completely circular face-on) and the planet is not required to be in transit, in front of the star, when measurements are taken.
Several objects believed to be exoplanets that are no longer gravitationally bound to a star have been photographed directly; however, the light from a planet’s host star is usually far too bright for the faint glow of reflected starlight from a planet to be distinguished in the visible part of the spectrum.
In 2010, however, scientists at NASA’s Jet Propulsion Laboratory showed that it was possible to block the light from a star through phase-shifting, using an optical vortex coronagraph, allowing orbiting planet around nearby stars to be directly imaged in the visible part of the spectrum for the first time.
Prior to this, some large Jupiter-like planets and brown dwarfs had also been resolved independently of their host stars, from heat given off in the infrared part of the spectrum, or from hydrogen-alpha frequency imaging.
The Theory of Relativity tells us that the path of light bends around massive objects. This can be used to detect planets as they pass in front of background stars or quasars, since the light from these distant objects will be distorted and magnified as if through a lens.