Planetary Science & Exoplanets: Worlds by the Thousands, Read from a Dip in Light

Look west after sunset on the right evening and a steady white point stands above the horizon. It does not twinkle like the stars around it. That is Venus, a world with a surface hot enough to melt lead, shining only because it reflects sunlight. Babylonian scribes logged its comings and goings on clay tablets, and the Maya built a calendar table around its 584-day cycle. For all of human history the planets were the five or six wanderers you could see, and a puzzle about the sky.

In October 1995 Michel Mayor and Didier Queloz announced a planet around a Sun-like star, 51 Pegasi. It was about half the mass of Jupiter and went round its star in 4.2 days. Nobody had expected such a thing, and they shared the 2019 Nobel Prize in Physics for it. As of 2 November 2026, the NASA Exoplanet Archive lists 6,451 confirmed planets. Almost none was seen as a picture. Most were found as a star dimming by a fraction of a per cent, or a star's speed changing by metres per second.

This piece is about how to read a world from almost nothing. It starts with the clock every orbit obeys, then asks how hot a planet is, what it is made of, where planets come from, how they are found, and what the catalogue does and does not tell us.

Kepler's third law: every orbit has a clock

Johannes Kepler published the third of his laws in 1619, after years of working through Tycho Brahe's measurements of Mars. It says that the farther a planet is from its star, the longer its year, and in a very particular way: the square of the period grows as the cube of the distance. Newton later showed why. Gravity weakens with the square of distance, and a heavier star pulls harder, so a planet around a heavy star goes round faster.

Worked: Neptune is 30.07 AU out. The cube is about 27,200, and its square root is 165, so Neptune takes about 165 years. That is correct: it completed its first lap since its 1846 discovery in 2011. Exoplanets use the same law backwards. A planet that crosses its star every 4.2 days, around a star of 1.1 Suns, must orbit at 0.053 AU, eight times closer than Mercury is to our Sun. That is how a world half the mass of Jupiter ended up cooking at the temperature of a lava lake.

Without JavaScript: Earth, 1 AU, 1 year. Jupiter, 5.2 AU, 11.9 years. Neptune, 30 AU, 165 years. A planet at 0.012 AU around a star of 0.09 Suns has a year of 1.5 days.

Notice the straight line on the log chart. A power law is a straight line when both axes are logarithmic, and the Solar System's planets sit on one. Change the star's mass and the whole line shifts, which is why a 1.5-day year around a dim red dwarf is not a contradiction.

Equilibrium temperature: sunlight in, heat out

A planet is a rock in a stream of light. It absorbs some, reflects the rest, and warms until it radiates away exactly as much as it takes in. That balance fixes a temperature, and the only inputs are the star's brightness, the distance and the reflectivity, called the albedo.

The fourth roots are the surprise. Move a planet twice as far away and the sunlight drops to a quarter, but the temperature drops only by the square root of two, to 71%. Worked for Earth: albedo 0.30 gives 278 × 0.915 = 255 K, or −18 °C. The real global mean is about 288 K. The 33 degrees between them is the greenhouse effect (see 13/15), and it is the difference between a frozen world and a living one.

Venus is the warning. With a thick cloud deck, its albedo is about 0.76, and the formula gives only 229 K, colder than Earth's. Its surface is about 737 K. The equilibrium temperature tells you what a bare, airless rock would do; the atmosphere can add anything from a few degrees to hundreds.

Without JavaScript: Earth 255 K without a greenhouse (288 K with); Venus 229 K from sunlight but 737 K at the surface; Mars about 210 K.

The so-called habitable zone is the band of distances where a rocky planet with an Earth-like atmosphere could have liquid water on its surface. Climate models that include how water vapour builds up put the Sun's zone at roughly 0.95 to 1.7 AU (Kopparapu and colleagues, 2013), which is starlight from about 1.1 down to 0.35 times Earth's. Those edges depend on the model, the atmosphere and the planet's spin. A planet in the zone is a candidate for an ocean, not a world known to have one.

Density: what a world is made of

You cannot weigh a planet on a scale, but you can weigh it by its pull on something else. Orbits give mass (Kepler again, for a moon or a neighbour), and a transit gives radius. Divide one by the volume of the other and you have the planet's mean density, which is the closest thing to a composition reading from a distance.

The numbers sort worlds at once. Water is 1. Rock is around 3. Iron is nearly 8 before pressure squeezes it. Earth is 5.5 because rock wraps an iron core. Mercury is 5.4 because it has a bigger core. Saturn is 0.69, less than water, and would float in a big enough bathtub. Neptune is 1.6, which tells you it is mostly water, ammonia and methane ices under a hydrogen skin.

The catch is that density is a single number and compositions are lists. A rocky planet with a deep ocean can match a rocky planet with a thick light atmosphere. Gravity compresses material, so a bigger planet is denser than the same stuff in a smaller one. The best honest statement is a range of mixtures. The other catch is the surprises. Kepler-51 d has about 5.6 Earth masses spread across about 9.5 Earth radii, giving 0.04 g/cm³, lighter than cork. Planets like it are called super-puffs, and how they form is open.

Without JavaScript: Earth 5.51 g/cm³, Mars 3.93, Neptune 1.64, Saturn 0.69, TRAPPIST-1 e about 4.9 (Earth-like), Kepler-51 d about 0.04.

Where planets come from: the snow line and the great migration

Planets are leftovers. A cloud of gas and dust collapses into a star, and the spinning disk around it clumps into grains, rocks, and then bodies the size of mountains and worlds. The Solar System's age comes from meteorites: the oldest dated grains, from primitive meteorites, give about 4.567 billion years, and the planets finished building within some tens of millions of years of that.

What a body can be made of depends on the temperature of the disk where it grows. Iron and silicates survive almost anywhere inside a few AU. Water freezes only beyond the snow line, near 2.7 AU for the young Sun. Beyond it, solid material is much more plentiful, so growing cores get big quickly, big enough to capture hydrogen and helium gas before the disk disperses. That is the classic picture of rocky planets inside and giants outside, and our Solar System fits it.

Without JavaScript: for the Sun, dust is at 170 K at 2.7 AU, so rock is available everywhere and water ice beyond about 2.7 AU.

Then exoplanets broke the tidy picture. 51 Pegasi b, a Jupiter-mass planet sitting 0.05 AU from its star, should not have formed there: it is far inside the snow line, where there is not enough solid material. The usual explanation is migration: planets form farther out and move inward, dragged by the disk or thrown by gravitational encounters. The Solar System may have had its own shuffle. The Nice model (Tsiganis and others, 2005) has Jupiter and Saturn crossing a resonance that rearranged the outer planets, and the Grand Tack model (Walsh and others, 2011) has Jupiter moving in and then back out. Both are active research. Neither is settled.

Another surprise appears in the radii. The Kepler data show a gap: far fewer planets have radii between about 1.5 and 2 Earth radii than just below or above (Fulton and others, 2017). Planets seem to come as bare rocky worlds or as rocky worlds wrapped in thick gas. Why the line falls there, and whether it comes from heat stripping atmospheres or from formation, is debated.

The transit: a shadow measured in parts per million

If a planet's orbit happens to line up with our line of sight, the planet crosses its star once per orbit and blocks a little of its light. The dip is the planet's shadow area over the star's area. A Jupiter-size planet crossing a Sun-size star hides about 1%. Earth hides 84 parts per million, about 0.008%. A telescope on the ground struggles below about 0.1%; space telescopes such as Kepler (launched 2009) and TESS (2018) reached tens of parts per million.

Two more things come free. The time between dips is the period, and with Kepler's law, the distance. The length of the dip depends on star size and orbit, and tells you the path the planet took across the star. The price is geometry: the orbit must be tilted just right. For a randomly tilted orbit the chance is about the star's radius divided by the orbit's radius, 0.47% for Earth around the Sun. For a hot Jupiter at 10 stellar radii it is 10%. That is why transit surveys need to watch tens of thousands of stars at once.

Without JavaScript: Earth in front of the Sun, a dip of about 100 ppm lasting 13 hours once a year, with a 0.5% chance of being seen at all. A hot Jupiter dips about 1.3 to 2% for 3 hours every 3.5 days.

Notice the red-dwarf button. A planet crossing a small star hides far more light, and its orbit at a given period is tighter, so both the dip and the odds improve. Most of the nearest rocky candidates are around red dwarfs for that reason. It is a statement about our instruments, not only about nature.

The wobble: a star's speed measured in metres per second

A planet does not orbit a motionless star. Both circle their common centre of mass, so the star traces a small circle too, and from far away you see its speed toward and away from you change. The change is read from the Doppler shift of lines in its spectrum, in the same way an ambulance siren shifts. 51 Pegasi b was found this way.

The dependence is sensible. A heavier planet tugs harder. A planet on a shorter orbit pulls faster. A heavier star is harder to move. Jupiter makes the Sun swing by 12.5 m/s, about the speed of a good sprinter, and Earth by 0.09 m/s, a slow crawl. A 12.5 m/s shift on a 500 nm line is a change of 4 parts in a hundred million, or 0.00002 nm. Detecting it means calibrating a spectrograph to a few parts in a billion.

The method has a built-in blind spot. You see only the motion along your line of sight, so what you measure is the planet's mass times the sine of the orbit's tilt, a minimum mass. A transit tells you the tilt is near 90°, and then the mass is the true mass. Combining the two methods gives mass and radius, so density, which is why the best-known planets are found by both.

Without JavaScript: Jupiter moves the Sun by 12.5 m/s, Earth by 0.09 m/s, and 51 Pegasi b moves its star by about 55 m/s.

The census is biased: what the catalogue can and cannot say

Now look at what has been found. The map below plots every confirmed planet in the NASA Exoplanet Archive that has a measured mass or minimum mass (3,229 of the 6,451), by its distance from its star and its mass. Choose a method and watch how the dots move. Transits and velocities crowd the top-left corner: big planets, close to their stars. Microlensing finds planets near 1 to 10 AU. Direct imaging finds only the heaviest planets, very far out. The bottom right, the place where our Earth and the other small planets of our Solar System would sit, is nearly empty.

Without JavaScript: of the 6,451 known planets, 4,785 were found by transits, 1,204 by radial velocity, 292 by microlensing and 98 by direct imaging.

The empty corner is not evidence that small, cool planets are rare. It is the shape of what our tools can do. A transit needs a lucky tilt and a tiny dip; a wobble needs a tiny speed; imaging needs a planet bright enough and far enough from its star to separate. Astronomers correct for this with careful statistics, and the corrected answer is that planets are common: most stars appear to have them, and small planets outnumber giants. How many Earth-size planets sit in habitable zones is still uncertain, with published estimates running from a few per cent to more than half of Sun-like stars, depending on the definition of "Earth-size" and "habitable".

And what counts as a planet at all? In August 2006 the International Astronomical Union voted at Prague that a planet orbits the Sun, is massive enough to be nearly round, and has cleared its orbit of other bodies. Pluto fails the third test and became a dwarf planet, along with Ceres and Eris. The IAU has no equally formal rule for exoplanets; its working guidance sets an upper limit near the mass where deuterium fusion starts, about 13 Jupiter masses.

Finally, the frontier. The James Webb Space Telescope reads starlight filtered through a planet's atmosphere as it crosses its star, picking out water, carbon dioxide and methane. Claims of biosignatures need more care than any other result in the field. In 2023 a team reported a possible dimethyl sulfide signal in the atmosphere of K2-18 b; later analyses by other teams did not find the signal statistically convincing, and the question remains open. A single gas rarely proves life, because chemistry that needs no life can often make it.

What a graduate just sees

• The square of a year is the cube of the distance, divided by the star's mass. Any period gives a distance.
• A planet's temperature goes as the fourth root of the sunlight and the inverse square root of distance, and the atmosphere changes it, sometimes enormously (Venus).
• Mass and radius give density, and density is the first clue to composition, though never the whole answer.
• Rock inside the snow line, giants outside it, and a lot of planets that moved.
• Depth is radius ratio squared; probability is star radius over orbit radius. Big, close planets are easy to find.
• A wobble gives only a minimum mass, until a transit supplies the tilt.
• Every catalogue is the product of its instruments. What is missing may be missing from the sky, or only from the telescope.

Where it shows up next

• Stars: Structure & Evolution (36/41): a planet's temperature and the width of its habitable zone move as its star brightens. In about a billion years the Sun's brightness will have raised Earth's temperature enough to end liquid oceans.
• Astronomical Data & Computation (36/44): transit and velocity signals are found by fitting models to noisy light curves, and TESS delivers more than anyone can look at by eye.
• Telescopes, Light & Spectra (36/39): the transmission spectrum is the same spectroscopy, applied to a planet's atmosphere.
• In the world: Mars rovers and sample return, asteroid missions such as OSIRIS-REx and Hayabusa2, and plans for telescopes that image Earth-size planets directly.

If you want to go deeper

• 📘 Textbook. Imke de Pater and Jack Lissauer, Planetary Sciences (Cambridge University Press, 2nd edition 2010; check for a newer edition), for the Solar System. Michael Perryman, The Exoplanet Handbook (Cambridge University Press, 2nd edition 2018), for detection methods.
• 📘 Gentler. Lisa Kaltenegger, Alien Earths (St. Martin's Press, 2024), on how astronomers search for habitable worlds.
• 🌐 Free data. The NASA Exoplanet Archive, where the numbers in this piece come from and where you can plot your own.
• 🌐 Free course. MIT OpenCourseWare 12.425 Extrasolar Planets: Physics and Detection Techniques (Fall 2007, Sara Seager).