How We Came to Know the Cosmos: Space & Time

Discover How We Came to Know the Cosmos

Chapter 28. Discovering Exoplanets

28.1 Early discoveries of exoplanets

Extrasolar planets (exoplanets) are planets that orbit stars other than the Sun. Viktor Safronov explained how stars and planets form in the 1970s with the solar nebular disc model (SNDM)[1] (discussed in Chapter 11). The first planets formed along with the second generation of stars since there were no heavy elements before this.

Giordano Bruno was the first person to suggest that there could be planets outside of the Solar System. Bruno was born 5 years after Nicolaus Copernicus published his heliocentric view of the universe, but this was presented as merely a mathematical system, not something to be taken literally.[2]

Bruno went further than Copernicus by stating that the Solar System really is heliocentric and that the stars really are other Suns that may also host planets with intelligent life. Bruno believed that space is infinite and stated that,

each sun is the centre of...many worlds which are distributed in as many distinct series in an infinite number of concentric and systems.[3]

Bruno didn’t think this contradicted the Church 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.[4]

Isaac Newton referenced exoplanets in the 1713 edition of the Principia,

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.[5]

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 involves measuring the precise location of objects.[6]

The American astronomer Thomas Jefferson Jackson “T. J. J.” See made another claim in 1896,[7] but this was disproven shortly after, and there’s currently no evidence that 70 Ophiuchi has planets.[8] The Dutch astronomer Peter van de Kamp claimed that there might be planets around the orange subgiant star, known as Barnard’s Star, in the 1960s,[9] but these detections were also shown to be false.[10]

The first observations of a real exoplanet were made in 1988 by the Canadian astronomers Bruce Campbell, Gordon Walker, and Stephenson Yang, using the radial velocity method.[11] 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 (discussed in Book II) 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 it’s now thought to be a gas giant about 1.6 times the mass of Jupiter.[12]

The first confirmed detection of an exoplanet came in 1992 when Aleksander Wolszczan and Dale Frail discovered three planets orbiting the pulsar PSR B1257+12, which is about 1000 light-years from Earth.[13] They did this using a method known as pulsar timing. PSR B1257+12 is about 1.4 times the mass of the Sun and is about 20 km wide, yet it rotates and hence pulsates over 160 times a second. Objects like planets can be discovered by measuring any changes in these pulsations, which may be caused by planets blocking them as they orbit.

PSR B1257+12’s three planets are named Draugr, Poltergeist, and Phobetor. Draugr has the lowest mass of any known planet and is about 0.02 times the mass as the Earth. Poltergeist and Phobetor 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 them would almost certainly have been destroyed by the supernova that created the pulsar.

The Swiss astronomers Michel Mayor and Didier Queloz discovered the first exoplanet orbiting a main sequence star in 1995, using the radial velocity method.[14] 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 the astronomer Rudolph Schild while working at the Harvard–Smithsonian Center for Astrophysics in 1996.[15] 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’s very unlikely to be confirmed.

In 2009, a team of Italian astronomers led 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.[16] Both of these planets were detected using gravitational microlensing (discussed in Chapter 5). Microlensing is an effect of general relativity, which shows that gravity bends spacetime and so affects the path of light.

28.2 Space-based projects

Data from the Hubble Space Telescope (HST) was used to discover 16 exoplanets in 2006 including SWEEPS-10.[17] SWEEPS-10 has one of the shortest known years, which is just 10 hours long.

A photograph of stars within the SWEEPS Field.

Figure 28.1
Image credit

SWEEPS Field, with planets highlighted.

Painting of an exoplanet.

Figure 28.2
Image credit

Artists’ impression of an exoplanet.

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.

The transit method detects planets by looking for the small drop in brightness that occurs when a planet transits - 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.

A plot of brightness against time showing that the brightness we observe lowers when a planet moves in front of a star and then increases again as it leaves. This is known as a transit light curve.

Figure 28.3
Image credit

A light curve produced by a planet crossing the path of a star.

Once the mass of a planet is known, scientists can work out what it's made of 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 orbit in the opposite direction.[18] Data from the HST has also been used to directly image planets, such as Fomalhaut b, which is about 25 light-years away.[19]

False-colour image of Fomalhaut b.

Figure 28.4
Image credit

False-colour composite image from the HST, showing the changing location of Fomalhaut b. The black circle blocks out the light from the star.

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.[20] Super-Earths are exoplanets with masses between about 1 and 15 times the mass of the Earth, where 15 times the mass of the Earth is the same as the mass of 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 (discussed in Chapter 25). Deep Impact stopped transmitting data back to Earth in 2013.[21]

NASA launched the Kepler spacecraft in 2009. This was designed to detect planets using the transit method and has discovered over 4000 new planetary candidates, over 2000 of which have already been confirmed.[22]

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.[23] 55 Cancri e is about 41 light-years away.

The ESA’s Gaia satellite was launched in 2013. Gaia began operating in 2014 and is expected to remain active until at least 2024. Data from Gaia are still being processed, but it is expected to detect thousands of planets using both the transit method and astrometry.[24]

NASA launched ASTERIA (Arcsecond Space Telescope Enabling Research in Astrophysics) in 2017 and TESS (the Transiting Exoplanet Survey Satellite) in 2018, both of which detect planets using the transit method.[25]

Future missions include the ESA and Swiss Space Office’s CHEOPS (CHaracterising ExOPlanets Satellite), which will examine known exoplanets to study how planets form [26] and the ESA's PLATO (Planetary Transits and Oscillations of stars) satellite. PLATO’s primary mission is to find habitable Earth-like planets using the transit method.[27]

Finally, the James Webb Space Telescope (JWST) should launch in 2021. 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 Hubble Space Telescope and should also be able to directly image planets.[28]

28.3 Ground-based projects

There are currently also about 40 ground-based projects that can 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 gravitational microlensing. Data from OGLE has been used to find about 20 exoplanets including super-Earth OGLE-2005-BLG-390Lb.[29]

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 the first exoplanet found to be orbiting a main sequence star, 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 its stellar system.[30]

A photograph of planet PSO J318.5-22.

Figure 28.5
Image credit

Exoplanet PSO J318.5-22, an image from the Pan-STARRS1 telescope.

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.[31]

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. PSR J1719-1438 b is about 3,900 light-years away.[32] It 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 almost 4000 confirmed exoplanets.[33][34] Most orbit stars similar in size to the Sun.[35][36] The stars with the most confirmed planets are the Sun and Kepler-90, which has at least eight planets.[37]

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.[38]

28.4 Habitable planets

Scientists 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 that the water doesn't boil but close enough that it doesn't freeze. Different stars have different temperatures, and so this region varies.[39] The hotter the star, the bluer it is, and the further the planet must be in order to be in the habitable zone.

A diagram showing the location of habitable zones for stars of three different sizes. The cooler the star, the closer the habitable zone.

Figure 28.6
Image credit

The hotter the star, the further the habitable zone.

It’s thought that one in five stars may contain habitable planets.[40] There may also be gas giants within the habitable zone that could have moons capable of hosting life.[41] The first Earth-sized planet to be confirmed to exist in the habitable zone was Kepler-186f, which orbits a red dwarf star.[42] Red dwarf stars emit red light, and so any potential plants that exist there would most likely be red instead of green.[43]

In 2016, astronomers at the European Southern Observatory (ESO) discovered that the red dwarf Proxima Centauri, the closest star to the Sun, hosts an Earth-sized planet in the habitable zone.[44] 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,[45] but this was disproved in 2015.[46] It’s estimated that about half of all planets may be in binary systems,[47] and other planets in binaries include Kepler-16b.[48]

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,[49] more precise observations could reveal the presence of chlorophyll or artificial compounds like CFCs.[50]

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 at the speed of light. Messages have already been sent to the Gliese 581 system, which contains at least three planets. These contain text composed by members of the public and translated into binary. They are due to arrive in 2029.[51]

A poster depicting Kepler-186 f in the style of a tourist destination. The poster states 'Kepler-186f, where the grass is always redder on the other side'.

Figure 28.7
Image credit

Space tourism poster for Kepler-186 f.

A poster depicting PSO J318.5-22 in the style of a tourist destination. The poster states 'PSO J318.5-22, where the nightlife never ends'.

Figure 28.8
Image credit

Space tourism poster for PSO J318.5-22.

28.5 References

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