1. The solar nebular disk model ↑
Extrasolar planets (exoplanets) are planets that orbit stars other than the Sun. Russian astronomer Viktor Safronov explained how stars and planets form in the 1970s, with the solar nebular disk model (SNDM).
The SNDM shows that stars and planets form from massive, dense clouds of gas. If the gas is made unstable by an outside force, like the shockwave from a supernova, then it may become 'clumpy', with more massive parts falling together under gravitation. The densest region will become a protostar, and eventually, a star.
The protostar rotates on its axis, and the rotational energy causes the rest of the cloud to flatten into a disc, known as a protoplanetary disc. The largest clumps in the protoplanetary disc become planets, and the smaller clumps become asteroids or comets.
The composition of these objects depends on their distance from the star, and hence their temperature. Objects close to the star are rocky because all of the water boils away, and objects further away are icy because all of the water freezes. It is not yet known exactly how gaseous planets form.
2. Early discoveries ↑
Italian philosopher Giordano Bruno was the first person to suggest that there could be planets outside of the Solar System. Bruno was born 5 years after Polish astronomer Nicolaus Copernicus published his heliocentric view of the universe, but this was presented as merely a mathematical system, not something to be taken literally.
Bruno went further than Copernicus, by stating that the Solar System really is heliocentric, and that the stars really are other Suns, which may also host planets and intelligent life. Bruno stated that in space, which he believed to be infinite:
"Each sun is the centre of...many worlds which are distributed in as many distinct series in an infinite number of concentric and systems".
Bruno did not think this contradicted Catholicism but he refused to believe in many other tenants of Catholicism. Instead, Bruno preferred Pantheism, the belief that the universe itself is divine. He was burnt at the stake for heresy in 1600.
"this most beautiful system of the sun, planets and comets, could only proceed from the counsel and dominion of an intelligent and powerful Being. And if the fixed stars are the centres of other like systems, these, being formed by the like wise counsels, must be all subject to the dominion of One; especially since the light of the fixed stars is of the same nature with the light of the sun, and from every system light passes into all other systems: and lest the systems of the fixed stars should, by their gravity, fall on each other mutually, [they] hath placed those systems at immense distances one from another".
The first claimed detections of exoplanets occurred in 1855, when Captain William Stephen Jacob, director of the Madras Observatory in India, suggested that the binary star system known as 70 Ophiuchi could host a planet. This was based on observations using astrometry, which deals with measuring the precise location of objects.
The first observations of a real exoplanet were made in 1988, by Canadian astronomers Bruce Campbell, Gordon Walker, and Stephenson Yang, using the radial velocity method. The radial velocity method detects changes in the rotational velocity of a star, with respect to the Earth, due to the mass of orbiting planets. The radial velocity is calculated using spectroscopy, and can be used to determine the minimum mass of a planet.
Campbell, Walker, and Yang detected the planet Gamma Cephei b. Gamma Cephei b is about 45 light-years from Earth and orbits the orange subgiant star, Gamma Cephei. Gamma Cephei b was not confirmed until 2003, and is now thought to be a gas giant about 1.6 times the mass of Jupiter.
The first confirmed detection came in 1992, when Polish astronomer Aleksander Wolszczan and Canadian astronomer Dale Frail discovered three planets orbiting the pulsar PSR B1257+12, which is about 1000 light-years from Earth. They did this using a method known as pulsar timing.
Pulsars are neutron stars, stars that have stopped nuclear fusion, and have shrunk in size after emitting most of their mass in a supernova. They radiate energy from their magnetic poles, which can sweep past the Earth as they rotate, creating pulsations of light.
PSR B1257+12 is about 1.4 times the mass of the Sun and is about 20 km wide, yet it pulsates, and hence rotates, over 160 times a second. Objects like planets can be discovered by measuring any changes in these pulsations.
PSR B1257+12's three planets are named PSR B1257+12 b, PSR B1257+12 c, and PSR B1257+12 d. PSR B1257+12 b has the lowest mass of any planet, and is about 0.02 times the mass as the Earth. PSR B1257+12 c and PSR B1257+12 d are about 4 times as massive as the Earth. These planets are most likely made of rock, ice, or iron, and any life that may have existed on these planets would almost certainly have been destroyed by the supernova that created the pulsar.
Swiss astronomers Michel Mayor and Didier Queloz discovered the first exoplanet orbiting a main sequence star in 1995, using the radial velocity method. This planet, known as 51 Pegasi b, is at least half as massive as Jupiter, and orbits a yellow, Sun-like star known as 51 Pegasi, which is 51 light-years away.
The first planet to be found outside of the Milky Way may have been discovered by astronomer Rudolph Schild, while working at the Harvard–Smithsonian Center for Astrophysics in 1996. This planet resides near NGC 3079, a galaxy about 50 million light-years away. However, this was observed in a chance event, and so it is very unlikely to be confirmed.
In 2009, an Italian team lead by Gabriele Ingrosso suggested that they may have found an exoplanet in the Andromeda galaxy, the closest spiral galaxy to our own, but this is also unlikely to be confirmed. Both of these planets were detected using microlensing.
Microlensing is an effect of general relativity, which shows that gravity bends spacetime, and so affects the path of light. This means that heavy objects can be found by the affect they have on the light of other objects, and so microlensing allows astronomers to detect faint objects.
3. Space-based projects ↑
Data from the Hubble Space Telescope (HST) was used to discover 16 exoplanets in 2006, including SWEEPS-10. This may be the planet with the shortest known year, which would be just 10 hours long, although it is still unconfirmed. These planets were found in the SWEEPS (Sagittarius Window Eclipsing Extrasolar Planet Search) survey, which monitored 180,000 stars for a week in order to detect extrasolar planets using the transit method.
SWEEPS Field, with planets highlighted. Image credit: NASA, ESA, K. Sahu (STScI) and the SWEEPS Science Team/Public domain.
The transit method detects planets by looking for the small drop in brightness that occurs when a planet passes in front of its star, with respect to the Earth. The size of a planet can be determined from the star's light curve and its mass can be found by combining this method with the radial velocity method.
Once the mass of a planet is known, its composition can be determined from its density. The composition of a planet's atmosphere can be determined using spectroscopy, and the temperature can be found by studying how the intensity of the host star changes as the planet orbits.
The easiest planets to find using the transit method are those that obscure the most light. These are known as hot-Jupiters. They are at least as large as Jupiter, and mostly orbit closer to their host star than Mercury. Hot-Jupiters are thought to have migrated from further out in the stellar system. Most do not orbit in the same plane as their host star, and some even orbit in the opposite direction.
False-colour composite image from the HST, showing the changing location of Fomalhaut b. The black circle blocks out the light from the star. Image credit: NASA, ESA, and P. Kalas (University of California, Berkeley and SETI Institute)/Public domain.
The first mission dedicated to finding exoplanets, CoRoT (COnvection ROtation and planetary Transits), was launched by the French Space Agency (CNES) and the European Space Agency (ESA) in 2006. CoRoT ceased to function in 2013, after being used to discover about 25 planets using the transit method, including 'super-Earth' COROT-7b. Super-Earth's are exoplanets with masses higher than the Earth, but below the mass of the least massive outer planet in the Solar System, Uranus.
In 2008, NASA's Deep Impact was used to study previously discovered exoplanets in a mission known as EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation). This was Deep Impact's second mission after visiting the comet Tempel 1 in 2005. Deep Impact stopped transmitting data back to Earth in 2013.
NASA launched the Kepler spacecraft in 2009. This was designed to detect planets using the transit method, and has found over 4000 new planetary candidates, over 2000 of which have been confirmed. Kepler is still in operation, despite a mechanical failure in 2013.
The Canadian Space Agency's (CSA) first space telescope, MOST (Microvariability and Oscillations of STars), has also been used to detect exoplanets, confirming the existence of super-Earth 55 Cancri e in 2011. 55 Cancri e is about 41 light-years away.
There are currently a number of missions to detect exoplanets. The first of these is Gaia, which was launched by the ESA in 2013. Gaia began operating in 2014, and is expected to remain active for at least 5 years. The first years' worth of data from Gaia is still being processed, but it is expected to detect thousands of planets using both the transit method and astrometry.
Future missions include the Swiss Space Office and the ESA's CHEOPS (CHaracterising ExOPlanets Satellite), and NASA's TESS (the Transiting Exoplanet Survey Satellite), which are due to launch in 2017. CHEOPS will examine known exoplanets in order to study how planets form, and TESS will study the brightest stars near the Earth, detecting planets using the transit method.
After a series of delays, the James Webb Space Telescope (JWST) is due to launch in 2018. The JWST is the result of a collaboration between NASA, the ESA, the CSA, and the Space Telescope Science Institute (STScI), which is the science operations centre for the HST. The JWST has been described as a successor to the HST, and should also be able to directly image planets.
Finally, the ESA plan to launch the PLATO (Planetary Transits and Oscillations of stars) satellite in 2024. PLATO's primary mission is to find habitable Earth-like planets using the transit method.
4. Ground-based projects ↑
There are currently also about 40 ground-based projects that have the ability to detect exoplanets. These include OGLE (the Optical Gravitational Lensing Experiment), the Geneva Extrasolar Planet Search, Pan-STARRS (the Panoramic Survey Telescope and Rapid Response System), and WASP (Wide Angle Search for Planets).
OGLE primarily uses the Las Campanas Observatory in Chile to look for dark matter using microlensing. Data from OGLE has been used to find about 20 exoplanets including super-Earth OGLE-2005-BLG-390Lb.
The Geneva Extrasolar Planet Search encompasses a variety of programs run by the Geneva Observatory in collaboration with several universities in Europe. The Geneva Extrasolar Planet Search was responsible for the discovery of 51 Pegasi b.
Pan-STARRS continually surveys the sky, imaging moving objects such as asteroids, comets, and variable stars. In 2013, Pan-STARRS discovered PSO J318.5-22, a planet that has no star. No one knows how planets come to exist without stars; however, PSO J318.5-22 might have been ejected from its protoplanetary disc in a collision during the formation of the stellar system.
WASP is an international organisation made up of eight universities that use an array of telescopes to look for planets. WASP has discovered about 100 planets using the transit method, including 'diamond planet' WASP-12b, which was discovered in 2008.
WASP-12b is about 870 light-years away, and orbits so close to its host star, a yellow dwarf, that it will be ripped apart in about 10 million years. It's carbon-rich, and may contain crystallised carbon - diamond.
Another 'diamond planet', PSR J1719-1438 b, was discovered around a pulsar in 2011, and is about 3,900 light-years away. PSR J1719-1438 b is thought to have originally been a star that later became a white dwarf - like the Sun will. Its close proximity to the pulsar meant that it lost most of its mass, leaving a planet-like core made of crystallised carbon and oxygen.
There are now over 2900 confirmed exoplanets in total. Over 10 are less than twice the mass of the Earth, and over 150 are less than the mass of Jupiter. Most orbit stars similar in size to the Sun and about 65% of planets are known to be in multi-planetary systems. The system with the most confirmed planets is KIC 11442793, which has at least seven planets.
Assuming that the small area of the Galaxy surveyed so far is representative of the rest of the disc of the Galaxy - and there is no reason to think that it isn't - then there should be about 100-400 billion exoplanets in the Milky Way, which contains about 300 billion stars, and at least 10% should be Earth-sized.
Kepler-186 f and PSO J318.5-22. Image credit: NASA/Public domain.
5. Habitable planets ↑
Scientists are most interested in finding planets that could contain life, and think that life is most likely to exist on planets that have liquid water on their surface. For this to be possible, a planet must orbit in a region known as the 'habitable zone', here the planet is far enough away from the star so that the water does not boil, but is close enough that it does not freeze. Different stars have different temperatures and so this region varies.
Habitable zones for different types of stars. Image credit: NASA/Public domain.
The hotter the star, the bluer it is, and the further the planet must be in order to be in the habitable zone. It's thought that one in five stars may contain habitable planets. There are also many gas giants within the habitable zone that could have moons capable of hosting life.
The first Earth-sized planet to be confirmed to exist in the habitable zone was Kepler-186f, which orbits a red dwarf star. Red dwarf stars emit red light, and so any potential plants that exist there would most likely be red instead of green.
In 2016, astronomers at the European Southern Observatory (ESO) discovered that the red-dwarf Proxima Centauri, the closest star to the Sun, contains an Earth-sized planet in the habitable zone.
Proxima Centauri is about 4.2 light-years away, and is gravitationally bound to the Alpha Centauri binary star system. This contains Alpha Centauri A, a sun-like G-type star, and Alpha Centauri B, an orange K-type star. Alpha Centauri B was briefly thought to contain a planet in 2012, but this was disproved in 2015. It's estimated that about half of all planets may be in binary systems, and other planets in binaries include Kepler-16b.
The next step in the search for life is to determine if planets in the habitable zone have atmospheres. If large amounts of methane and oxygen were found, then this would suggest that something is continually producing these gases, like plants do on Earth. Although this is not proof of life, more precise observations could reveal the presence of chlorophyll or artificial compounds like CFCs.
If we do find signs of life on another planet, then we could explore using robotic probes, or try to communicate by sending them a message, which would travel the speed of light. Messages have already been sent to the Gliese 581 system, which contains three planets. These contain text composed by members of the public and translated into binary, and are due to arrive in 2029.