Field · Emerged 1992 – 2009

Exoplanetary Science

Are there planets around other stars, what are they like, and could any of them host life?

4 chapters3 min read6 turning points 1 open problem

Branched from: Astronomical Spectroscopy + Stellar Astrophysics
Branched into: Not yet surveyed past here
Figures: Aleksander Wolszczan, Dale Frail, Michel Mayor, Didier Queloz, David Charbonneau, Gregory Henry, William Borucki, Michaël Gillon

In brief

Exoplanetary science finds and studies planets orbiting stars other than the Sun. They are too faint and too close to their stars to photograph easily, so almost all are detected indirectly: by the tiny wobble a planet's gravity causes in its star, or by the slight dimming as a planet crosses the star's face.

Whether other planetary systems exist was debated for centuries. The first confirmed planets, found in 1992 and 1995, were bizarre: worlds orbiting a dead star, and a giant planet circling its star in four days. Space telescopes then showed that planets are the rule rather than the exception. More than six thousand are now known, and telescopes have begun to read the chemistry of their atmospheres.

Key ideas

Radial velocity methodEnters 1995: A planet and its star orbit their common centre of mass, so the star wobbles. The wobble shifts the star's spectral lines back and forth, revealing the planet's period and a minimum mass.
Transit methodEnters 1999 – 2002: When a planet crosses in front of its star, the star dims by the fraction of its disc the planet covers. The depth gives the planet's size and the timing its orbit.
Hot JupiterEnters 1995: A giant planet orbiting very close to its star, in days rather than years. Theories said none should exist, and their discovery showed that planets migrate.
Habitable zoneEnters 2017: The range of distances from a star where a rocky planet could have liquid water on its surface. It depends on the star's brightness.
Transmission spectroscopyEnters 2022: During a transit, some starlight passes through the planet's atmosphere, which absorbs at the wavelengths of its molecules. The atmosphere's chemistry can be read from the difference.

Chapter I

Other Worlds

Whether other suns have planets has been argued since the ancient Greeks, and Giordano Bruno, who held that they do, was burned for heresy in 1600. For most of the twentieth century it was assumed they must, but no one could see them. A planet is a billion times fainter than its star and, seen from light-years away, almost touching it. Several claimed detections turned out to be wrong.

The first confirmed planets were a surprise. In 1992 Aleksander Wolszczan and Dale Frail found small planets orbiting a pulsar, a neutron star left from a supernova, betrayed by tiny regularities in the arrival times of its pulses. They could not be home to life, and their existence was hard to explain.

Chapter II

A Planet Where None Should Be

In 1995 Michel Mayor and his student Didier Queloz found a planet around the Sun-like star 51 Pegasi. Spectroscopy showed the star's lines shifting back and forth every 4.2 days, as the star wobbled at about 60 metres per second. The companion was half as massive as Jupiter and orbited about seven times closer to its star than Mercury is to the Sun. Theories of planet formation, built on our own solar system, said giant planets form far out. Planets, it emerged, migrate.

In 1999 David Charbonneau and Gregory Henry, working in separate teams, saw a planet pass in front of its star, dimming it slightly. The transit method gives a planet's size, and combined with the wobble, its density. In 2009 NASA launched Kepler, the mission William Borucki had proposed again and again from 1992 onwards. It watched 150,000 stars and found thousands of planets. Most stars have planets, and small rocky ones are common.

Chapter III

A Closer Look: How Faint Is a Planet's Signal?

Transits. A planet crossing its star blocks a fraction of the star's light equal to the ratio of their disc areas, $(R_{\text{planet}}/R_{\text{star}})^2$ . For Jupiter crossing the Sun, with radii of 69,900 km and 695,700 km:

\left(\frac{69{,}900}{695{,}700}\right)^2 \approx 0.010 ,

a 1% dip. For the Earth, radius 6,371 km, it is

\left(\frac{6{,}371}{695{,}700}\right)^2 \approx 0.000084 ,

84 parts per million, like a flea crossing a car headlight. That is why finding Earths needs a telescope in space, above the twinkling atmosphere, watching the same stars for years.

Wobbles. The star and planet orbit their common centre of mass, so the star moves at the planet's orbital speed times the ratio of their masses. Jupiter orbits at 13.1 km/s and has about $1/1050$ of the Sun's mass, so it makes the Sun wobble at

13.1 \text{ km/s} \times \frac{1}{1050} \approx 12.5 \text{ m/s} ,

about the speed of a sprinter. The Earth, orbiting at 29.8 km/s with $1/333{,}000$ of the Sun's mass, moves it at only

29.8 \text{ km/s} \times \frac{1}{333{,}000} \approx 9 \text{ cm/s} ,

about the pace of a walking tortoise. 51 Pegasi b made its star wobble at about 60 m/s because it is massive and very close, which is exactly why it was the first to be found. Detecting a true Earth twin by its wobble needs spectrographs stable to a few centimetres per second over years, and that is only now being attempted.

Chapter IV

Reading Atmospheres

The newest step is chemistry. When a planet transits, a little starlight filters through its atmosphere, and the molecules there leave their spectral fingerprints. In 2022 the James Webb Space Telescope detected carbon dioxide in a giant planet's atmosphere. In 2017 Michaël Gillon's team found seven Earth-sized planets around TRAPPIST-1, a nearby dwarf star, several at temperatures where water could be liquid. The ultimate goal is a signature of life, such as oxygen alongside methane, on a rocky planet. That would bear on the deepest open problem of evolutionary biology: how, and how often, life begins.

Turning points6

1992DISCOVERY
Planets around a dead star
Aleksander Wolszczan and Dale Frail, timing the millisecond pulsar PSR B1257+12 with the Arecibo radio telescope, find regular variations in its pulses caused by two planets a few times the Earth's mass. They were the first confirmed planets outside the solar system, orbiting the remnant of a supernova.
Aleksander Wolszczan, Dale Frail
› Sources (1)
- Wolszczan, A. & Frail, D. A. (1992). A planetary system around the millisecond pulsar PSR1257+12. Nature 355: 145–147.
1995DISCOVERYContested
51 Pegasi b
Michel Mayor and Didier Queloz, measuring stellar wobbles with a spectrograph in southern France, find a planet about half Jupiter's mass orbiting the Sun-like star 51 Pegasi every 4.2 days, far closer than Mercury orbits the Sun. No theory predicted such a planet. It was the first planet found around a Sun-like star, and won the 2019 Nobel prize.
What is disputed
Earlier claims complicate the priority. Bruce Campbell, Gordon Walker and Stephenson Yang reported evidence in 1988 of a planet around Gamma Cephei, but withdrew it. It was confirmed in 2003. David Latham's 1989 companion to HD 114762 was long counted as a possible planet but later shown to be far more massive. Mayor and Queloz's detection was the first around a Sun-like star that was clear, confirmed and accepted.
Michel Mayor, Didier Queloz
› Sources (2)
- Mayor, M. & Queloz, D. (1995). A Jupiter-mass companion to a solar-type star. Nature 378: 355–359.
- Campbell, B., Walker, G. A. H. & Yang, S. (1988). A search for substellar companions to solar-type stars. Astrophysical Journal 331: 902–921.
1999 – 2002EXPERIMENT
A planet crosses its star
David Charbonneau and colleagues, and independently Gregory Henry's team, see the star HD 209458 dim by about 1.7% every 3.5 days as a planet passes in front of it. The transit gave the planet's size and, with its wobble, its density: a gas giant. In 2002 sodium was detected in its atmosphere, the first chemistry of an exoplanet.
David Charbonneau, Gregory Henry
› Sources (2)
- Charbonneau, D., Brown, T. M., Latham, D. W. & Mayor, M. (2000). Detection of planetary transits across a Sun-like star. Astrophysical Journal 529(1): L45–L48.
- Charbonneau, D., Brown, T. M., Noyes, R. W. & Gilliland, R. L. (2002). Detection of an extrasolar planet atmosphere. Astrophysical Journal 568(1): 377–384.
2009 – 2018EXPERIMENT
The Kepler survey
NASA's Kepler space telescope stares at about 150,000 stars for four years, watching for transits. It finds thousands of planets and shows that small planets are common, and that most Sun-like stars have planets. Kepler, conceived by William Borucki, who proposed it repeatedly from 1992, turned planets from rarities into a population.
William Borucki
› Sources (1)
- Borucki, W. J. et al. (2010). Kepler planet-detection mission: introduction and first results. Science 327(5968): 977–980.
2017DISCOVERY
Seven Earth-sized planets
Michaël Gillon's team finds seven Earth-sized planets transiting TRAPPIST-1, a small, cool star forty light-years away, several of them in its habitable zone. Because the star is small, the planets block a large fraction of its light, making their atmospheres among the most accessible of any rocky planets.
Michaël Gillon
› Sources (1)
- Gillon, M. et al. (2017). Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1. Nature 542: 456–460.
2022EXPERIMENT
JWST reads a planet's atmosphere
Among its first observations, the James Webb Space Telescope detects carbon dioxide in the atmosphere of the hot gas giant WASP-39b, then sulphur dioxide made by photochemistry. Exoplanet atmospheres could now be analysed in detail, molecule by molecule.
› Sources (1)
- JWST Transiting Exoplanet Community Early Release Science Team (2023). Identification of carbon dioxide in an exoplanet atmosphere. Nature 614: 649–652.

Applications

Where it is used

Astrobiology↗ Biology · Evolutionary Biology
Is Earth's life unusual?
The study of life's origin now has thousands of other planets to compare with Earth. Rocky planets in habitable zones turn out to be common, which sharpens the question of how easily life begins, the open problem at the root of evolutionary biology.
› Sources (1)
- Schwieterman, E. W. et al. (2018). Exoplanet biosignatures: a review of remotely detectable signs of life. Astrobiology 18(6): 663–708.
Instrumentation
Laser frequency combs
Detecting an Earth's wobble on a Sun-like star needs spectrographs calibrated to centimetres per second. Laser frequency combs, developed for atomic clocks, now provide rulers of light precise enough for the task.
› Sources (1)
- Steinmetz, T. et al. (2008). Laser frequency combs for astronomical observations. Science 321(5894): 1335–1337.

Open problems

Where the map runs out

Open

Is there life on other planets?

Open as of 2026. No biosignature has been confirmed.

Life on Earth has changed its atmosphere, filling it with oxygen and methane that would not otherwise coexist. Could a telescope detect such a signature on a planet light-years away, and would it be convincing?

Why it is hard: Earth-like planets around Sun-like stars are too faint and too close to their stars for current telescopes to analyse. Every candidate gas can also be produced without life, so a detection must rule out geological and chemical explanations. Recent claims, such as dimethyl sulphide on K2-18b, have been disputed.
What resolving it unlocks: An answer to one of the oldest questions: whether life is unique to Earth. Even a clear negative, over many planets, would say how rare life is.

› Sources (1)

Schwieterman, E. W. et al. (2018). Exoplanet biosignatures: a review of remotely detectable signs of life. Astrobiology 18(6): 663–708.