Nobel Prize in Physics 2019: Cosmic Evolution and the First Exoplanet

This note covers the Nobel Prize in Physics 2019: who won it, how James Peebles' theoretical work explains the evolution of the universe, how Michel Mayor and Didier Queloz discovered the first exoplanet around a sun-like star, the methods behind both discoveries, how the science unfolded, why it matters and quick facts for exams.

What was the Nobel Prize in Physics 2019 awarded for?

The Royal Swedish Academy of Sciences awarded the prize with the citation: "for contributions to our understanding of the evolution of the universe and Earth's place in the cosmos".

In plain words, one scientist worked out how the whole universe grew and changed after the Big Bang, while two others found the first planet circling a star like our Sun outside our solar system.

The committee split the prize into two separate contributions that were linked by a common theme: understanding where we are in the universe. One half went to James Peebles for building the theory that explains the universe's structure and history.

The other half, shared equally, went to Michel Mayor and Didier Queloz for discovering an exoplanet, a planet orbiting a star other than the Sun.

The official name of this award is the Nobel Prize in Physics, and it is one of the original five prizes created by Alfred Nobel's will.

Although the two discoveries look very different, one is about the whole cosmos and the other about a single distant planet, the Academy linked them because both reshaped how scientists think about Earth's place in the cosmos: Peebles showed that ordinary matter, the stuff we are made of, is only a tiny fraction of everything that exists, while Mayor and Queloz showed that planetary systems like ours are not rare freaks of nature but widespread across the galaxy.

Who are the laureates?

James Peebles

James Peebles was born on 25 April 1935 in Winnipeg, Canada. He studied at the University of Manitoba before moving to the United States, where he earned his Ph.D. in 1962 from Princeton University.

At the time of the award he held the position of Albert Einstein Professor of Science at Princeton University, USA. He received one half of the prize.

His contribution, in the committee's words, was "theoretical discoveries in physical cosmology". Starting in the mid-1960s he built a mathematical framework that let scientists interpret the faint afterglow radiation left over from the Big Bang, and later proposed that most of the universe's matter and energy is in forms we cannot directly see, now called dark matter and dark energy.

Michel Mayor

Michel Mayor was born on 12 January 1942 in Lausanne, Switzerland. He studied at the University of Lausanne and then the Geneva Observatory, earning his doctorate in astronomy in 1971 and becoming a professor there in 1988.

At the time of the award his affiliation was the University of Geneva, Geneva, Switzerland. He received one quarter of the prize.

His citation reads "for the discovery of an exoplanet orbiting a solar-type star". He led the team that built the custom spectrograph used to detect the first such planet, and he had spent many years before that constructing instruments to measure tiny wobbles in stars caused by orbiting companions.

Didier Queloz

Didier Queloz was born on 23 February 1966 in Geneva, Switzerland. He studied at the University of Geneva, where Michel Mayor supervised his doctoral work, completed in 1995.

At the time of the award he was affiliated with both the University of Geneva, Switzerland and the University of Cambridge, United Kingdom. He received one quarter of the prize, sharing the same citation as Mayor.

As a doctoral student, Queloz was asked to help develop new, more precise measuring methods for the research group. These refinements in instrument design were essential to actually catching the signal of the planet that the two scientists announced together in 1995.

What problem were these discoveries trying to answer?

Before the 1960s, cosmology, the study of the universe's origin and history, was described by the Nobel materials as largely speculation rather than hard science.

Astronomers knew the universe was expanding, following Edwin Hubble's observations in 1929, but there was little detailed, testable theory connecting the Big Bang to the structures, galaxies and stars we see today.

A second, separate question had puzzled thinkers for centuries: are there other worlds like Earth orbiting other suns? Early attempts to detect such planets began decades before 1995, but the instruments were not sensitive enough.

The nobelprize.org popular-science account notes that in the early 1990s, the search for exoplanets "was not part of mainstream astronomy", reflecting how uncertain scientists were that any such planet could even be found.

Both strands of work needed exceptionally precise measurement. For cosmology, scientists needed to detect microscopic temperature variations in radiation left over from the universe's infancy.

For exoplanets, scientists needed to detect star movements of only a few metres per second, far smaller than ordinary stellar motion, caused by the gentle gravitational tug of an orbiting planet.

How did Peebles' cosmology work?

The universe, according to the Big Bang model described in the sources, began almost 14 billion years ago in an extremely hot, dense state. As it expanded, it cooled.

Roughly 400,000 years after the Big Bang, the temperature had dropped enough for electrons and nuclei to combine into atoms, and light was finally able to travel freely through space.

This ancient light still fills the cosmos today as the cosmic microwave background, often shortened to CMB.

Peebles' key insight was realising that tiny variations hidden in this background radiation carry information about how matter was distributed in the very early universe, information that later shaped how galaxies and galaxy clusters formed. His theoretical steps, built from the mid-1960s onward, can be summarised as follows:

1. Work out, from the physics of the hot early universe, what the temperature and structure of the leftover background radiation should look like today.
2. Show how small temperature variations in that radiation connect mathematically to the amount of matter and energy present at the time.
3. Propose, in 1982, that a form of invisible, slow-moving matter, now called cold dark matter, was needed to let structures such as galaxies clump together as observed.
4. Reintroduce, in 1984, Einstein's abandoned cosmological constant as a form of energy filling empty space, now called dark energy, to balance the equations describing a flat universe.

The outcome of this framework was striking: ordinary matter, the stuff that makes stars, planets and people, accounts for only about 5 per cent of the universe's total content.

Dark matter makes up another slice, and the largest share, dark energy, makes up the rest.

The press release states the split as "just five per cent of its content is known", with the remaining 95 per cent unknown dark matter and dark energy.

Draw a horizontal line representing time from the Big Bang on the left to today on the right.

Mark the moment about 400,000 years in when the universe became transparent and light began travelling freely; label the leftover light reaching us today as the cosmic microwave background, now cooled close to absolute zero.

How did Mayor and Queloz detect an exoplanet?

Planets do not glow brightly enough on their own to be seen directly next to their much brighter host star.

Instead, Mayor and Queloz used an indirect method called the radial velocity method, which relies on the fact that a star and its orbiting planet both move around a shared centre of gravity, causing the star to wobble very slightly.

This wobble changes the colour of the star's light slightly through the Doppler effect: light shifts towards blue as the star moves towards Earth and towards red as it moves away, the same effect that changes the pitch of an ambulance siren as it passes. The process the team followed was:

1. Build an extremely sensitive spectrograph able to split starlight into thousands of wavelengths at once.
2. Point the instrument at the star 51 Pegasi using custom instruments at the Haute-Provence Observatory in southern France.
3. Measure tiny repeating shifts in the star's light wavelengths over time, revealing a regular wobble.
4. Calculate the planet's orbital period and a lower-limit estimate of its mass from the pattern and strength of that wobble.

The planet they found, named 51 Pegasi b, surprised everyone. It is a gas giant comparable in size to Jupiter, yet it orbits its star in just four days, at a distance of only about eight million kilometres, heating it to over 1,000°C.

This contradicted existing theory, which expected Jupiter-sized planets to form and stay far from their star, the way Jupiter itself takes almost 12 years to circle the Sun.

Draw a star and a planet orbiting a shared centre of gravity between them; show the star tracing a small circular wobble, and mark its light as bluer when moving towards an observer on Earth and redder when moving away.

How did the discovery unfold?

This timeline shows two separate stories running side by side for decades before they were finally recognised together. Peebles' theoretical ideas, built up gradually from the mid-1960s, took many years to be confirmed by observation; his cold dark matter proposal of 1982 and his revival of the cosmological constant in 1984 were only fully supported by satellite measurements and the 1998 discovery of accelerating expansion, long after he first put the ideas forward.

Meanwhile, Mayor spent nearly two decades refining his instruments, starting with his first spectrograph in 1977, before Queloz's contributions as a doctoral student finally pushed the precision far enough to catch the tiny wobble caused by 51 Pegasi b. Both stories show how patient, incremental improvement in theory and instruments eventually produced a breakthrough.

Why does it matter?

Peebles' framework turned cosmology from guesswork into precision science. The nobelprize.org materials say his theoretical tools let scientists measure the universe's age, composition and fate with real accuracy, even though 95 per cent of its content, dark matter and dark energy, remains unexplained.

This is a mystery and a challenge to modern physics, in the committee's own words.

The discovery of 51 Pegasi b sparked what the sources call a revolution in astronomy.

Since 1995, more than 4,000 exoplanets have been found in the Milky Way, showing an enormous variety of sizes, orbits and compositions that forced scientists to rethink how planetary systems form.

Instruments and missions such as the Kepler Space Telescope and TESS continue the search, sometimes using a second method called transit photometry alongside the radial velocity approach.

Open questions remain on both fronts: nobody yet knows what dark matter particles actually are, why dark energy has the value it does, or whether any of the thousands of known exoplanets host life.

The sources describe this as an ongoing search, with new projects planned to keep hunting for answers.

How does this connect to what you study?

The physics of light and waves taught in school, such as the Doppler effect used to explain how an ambulance siren changes pitch as it passes, is exactly what Mayor and Queloz used to detect a wobbling star. Light shifting towards blue or red as a source moves towards or away from an observer is the same underlying principle, whether applied to sound waves or to starlight.

Similarly, basic ideas about gravity and orbits taught in school, the fact that two bodies pull on each other and orbit a shared centre of mass rather than one simply circling the other, underpin how astronomers infer a planet's existence without ever seeing it directly. A star and its planet both move, however slightly.

Cosmology topics such as the Big Bang, the expansion of the universe and the existence of invisible dark matter and dark energy also connect to school science discussions of the scale and structure of the universe. Ideas such as atoms forming from a hot particle soup as the universe cooled, described in the sources as happening about 400,000 years after the Big Bang, build on basic atomic structure concepts taught at school level, even though the full mathematics of general relativity used by Peebles goes well beyond what is taught in school.

Students who study spectra in chemistry or physics, splitting light into its component wavelengths, are using the same basic tool, a spectrograph, that Mayor and Queloz depended on to measure starlight precisely enough to find 51 Pegasi b.

Quick facts for exams

The Nobel Prize in Physics 2019 was announced on 8 October 2019 by the Royal Swedish Academy of Sciences.

It was shared between James Peebles, who won one half for theoretical discoveries in physical cosmology explaining the universe's evolution, and Michel Mayor and Didier Queloz, who shared the other half for discovering the first exoplanet orbiting a sun-like star in 1995.

Peebles worked at Princeton University in the USA, while Mayor and Queloz were based at the University of Geneva in Switzerland, with Queloz also linked to the University of Cambridge in the UK.

The total prize amount was 9,000,000 Swedish kronor, divided according to each laureate's share.

Note: Source. The prize facts in this note are from the Nobel Prize's official site, nobelprize.org.

Glossary

• Cosmology — the scientific study of the universe's origin, structure and long-term evolution.
• Big Bang — the model describing the universe's origin from an extremely hot, dense state almost 14 billion years ago.
• Cosmic microwave background (CMB) — the faint radiation left over from the early universe, now visible as microwaves filling all of space.
• Dark matter — an invisible form of matter detected only through its gravitational effects, needed to explain how galaxies hold together.
• Dark energy — an unknown form of energy filling empty space, linked to the accelerating expansion of the universe.
• Cosmological constant — a term Einstein added to his equations, later linked to dark energy.
• Exoplanet — a planet that orbits a star other than the Sun.
• Solar-type star — a star with properties similar to the Sun.
• Radial velocity method — a technique that detects a planet by measuring the wobble of its host star through Doppler shifts.
• Doppler effect — the change in wavelength of light or sound caused by relative motion between source and observer.
• Spectrograph — an instrument that splits light into its component wavelengths for precise measurement.
• Transit photometry — a method that detects a planet by measuring the dip in a star's brightness as the planet passes in front of it.
• 51 Pegasi b — the first exoplanet discovered orbiting a solar-type star, announced in 1995.

Common errors and misconceptions

• Misconception: Mayor and Queloz saw the exoplanet directly through a telescope. Correct: they detected it indirectly, through tiny wobbles in the star's light caused by the planet's gravity.
• Misconception: dark matter and dark energy are the same thing. Correct: dark matter is invisible matter that pulls through gravity, while dark energy is linked to the accelerating expansion and pushes space apart.
• Misconception: 51 Pegasi b is similar to Earth. Correct: it is a gas giant comparable to Jupiter, but orbiting far closer to its star than any planet in our solar system.
• Misconception: the whole 2019 physics prize was about exoplanets. Correct: one half went to Peebles for cosmology theory, and the other half jointly to Mayor and Queloz for the exoplanet discovery.
• Misconception: scientists know what dark matter is made of. Correct: its existence is inferred from gravitational effects, but its exact composition remains unknown.
• Misconception: the cosmic microwave background was predicted and then immediately found by the same people. Correct: it was first detected by chance by Penzias and Wilson, and only later understood through theoretical work including Peebles'.

Exam-style questions with model answers

Q1. In which year was the Nobel Prize in Physics 2019 announced? [1 mark]

1. It was announced on 8 October 2019 by the Royal Swedish Academy of Sciences.

Q2. Name the exoplanet discovered by Michel Mayor and Didier Queloz. [2 marks]

1. They discovered 51 Pegasi b, a gas giant planet orbiting the star 51 Pegasi, announced in October 1995.

Q3. Explain what the radial velocity method measures and why it was needed to find 51 Pegasi b. [4 marks]

1. A planet does not glow brightly enough to be seen directly beside its host star, so astronomers use an indirect method. The radial velocity method measures the tiny wobble a star makes as it and its orbiting planet move around their shared centre of gravity. This wobble shifts the star's light slightly towards blue when moving towards Earth and towards red when moving away, an effect caused by the Doppler shift. By measuring these repeating colour shifts precisely, astronomers can work out the planet's orbital period and estimate its mass, even without seeing the planet itself.

Q4. What share of the prize did each laureate receive, and for what contribution? [4 marks]

1. James Peebles received one half of the prize for theoretical discoveries in physical cosmology, explaining how the universe evolved after the Big Bang and introducing ideas such as cold dark matter and the cosmological constant. Michel Mayor and Didier Queloz shared the other half, one quarter each, for discovering 51 Pegasi b, the first exoplanet found orbiting a solar-type star, using a radial velocity spectrograph they built in Switzerland.

Q5. Discuss how James Peebles' theoretical work changed our understanding of the universe's composition. [6 marks]

1. Before Peebles' work, cosmology lacked a detailed, testable framework connecting the Big Bang to the universe we observe today. Starting in the mid-1960s, Peebles developed theoretical tools to interpret the cosmic microwave background, the faint radiation left over from about 400,000 years after the Big Bang, when the universe first became transparent to light. He showed that tiny variations in this radiation carry information about how matter was distributed in the early universe, shaping the later formation of galaxies and galaxy clusters. In 1982, he proposed that cold dark matter, a slow-moving, invisible form of matter, was necessary for structures to form as observed. In 1984, he reintroduced Einstein's abandoned cosmological constant as a form of energy filling empty space, now called dark energy, to make the equations describing a flat universe balance correctly. Together these ideas revealed that ordinary matter, the stuff making up stars, planets and people, is only about 5 per cent of the universe's content, with the remaining 95 per cent made up of unknown dark matter and dark energy, a mystery that still challenges modern physics.

Q6. 51 Pegasi b orbits its star in just four days at a distance of only about eight million kilometres, while Jupiter takes almost 12 years to orbit the Sun. Why was this considered surprising at the time? [3 marks]

1. 51 Pegasi b is a gas giant comparable in size to Jupiter, yet it orbits its star in just four days at a distance of only about eight million kilometres. Existing theory expected such large planets to form far from their star, similar to Jupiter's nearly twelve-year orbit around the Sun, so this close, fast orbit challenged accepted ideas about planet formation.

Q7. State the official citation for the Nobel Prize in Physics 2019. [2 marks]

1. The citation was "for contributions to our understanding of the evolution of the universe and Earth's place in the cosmos".

Q8. What role did Didier Queloz play as a doctoral student in the exoplanet discovery? [3 marks]

1. As a doctoral student under Michel Mayor's supervision at the University of Geneva, Queloz was asked to help develop new, more precise instruments and methods for measuring stellar wobbles. These refinements in spectrograph precision were essential to detecting the faint signal produced by 51 Pegasi b.

Key takeaways

• The Nobel Prize in Physics 2019 was split between cosmology theory and exoplanet discovery, both about Earth's place in the cosmos.
• James Peebles built the theoretical framework that lets scientists interpret the cosmic microwave background and model the universe's history.
• Ordinary matter makes up only about 5 per cent of the universe; the rest is unknown dark matter and dark energy.
• Michel Mayor and Didier Queloz discovered 51 Pegasi b in 1995, the first exoplanet found orbiting a solar-type star.
• The radial velocity method detects planets indirectly, through Doppler shifts caused by a star's gravitational wobble.
• More than 4,000 exoplanets have since been found, revealing great diversity in planetary systems.
• Peebles worked at Princeton University; Mayor and Queloz were based at the University of Geneva and Cambridge.
• Many open questions remain, including the true nature of dark matter and dark energy and whether other life exists.

Test yourself

Where was James Peebles born, and where was he working at the time of the award?

James Peebles was born in Winnipeg, Canada, and at the time of the award he worked at Princeton University in the USA.

What fraction of the Nobel Prize in Physics 2019 did Michel Mayor receive?

Michel Mayor received one quarter of the prize, sharing it equally with Didier Queloz for their exoplanet discovery.

What is the cosmic microwave background?

It is the faint leftover radiation from the universe's early hot phase, released when the universe first became transparent to light.

Name the first exoplanet discovered orbiting a solar-type star.

The planet is called 51 Pegasi b, discovered orbiting the star 51 Pegasi, announced in October 1995.

What effect do Mayor and Queloz's instruments rely on to detect a star's wobble?

They rely on the Doppler effect, which shifts a star's light towards blue or red depending on its motion towards or away from Earth.

What two unknown components make up most of the universe's content, according to Peebles' framework?

They are dark matter and dark energy, which together make up about 95 per cent of the universe's content.

How many exoplanets had been found in the Milky Way by the time of this prize announcement?

According to the sources, more than 4,000 exoplanets had been found in the Milky Way by 2019.