How astronomers find planets around other stars

For most of human history, planets were objects we could see only in our own Solar System. That changed in 1992, when astronomers confirmed planets around a pulsar, and in 1995 when 51 Pegasi b was found around a Sun-like star. Since then, the number of known exoplanets has grown into the thousands. Two methods have done most of the work: the transit method and the radial velocity method.

The transit method watches for a tiny drop in starlight

If a planet passes in front of its star from our point of view, it blocks a small fraction of the light. Astronomers call this a transit. The size of the dip tells them the planet’s size compared with the star.

For a planet like Earth crossing the Sun, the drop in brightness would be about 0.008 percent, or 84 parts per million. That is a very small signal. For Jupiter crossing the Sun, the drop would be about 1 percent, which is much easier to detect. A Saturn-sized planet gives a drop near 0.8 percent.

The dip repeats once per orbit, so transits also reveal the planet’s period. If a planet transits every 10 days, then its year is 10 days long. Kepler, NASA’s space telescope launched in 2009, used this idea to find thousands of candidates by watching more than 150,000 stars at once. The later TESS mission, launched in 2018, has looked at almost the whole sky in wide fields and has found many planets around bright nearby stars.

The transit method has limits. The star and planet must line up almost edge-on as seen from Earth. For a planet orbiting at about 1 astronomical unit around a Sun-sized star, the chance of a transit is only about 0.5 percent. Close-in planets are much easier to catch because their orbits are smaller. A planet at 0.05 AU has a transit probability near 10 percent.

Transits can also give hints about a planet’s atmosphere. As the planet crosses the star, a tiny amount of starlight passes through the planet’s atmosphere. Different gases absorb different wavelengths. That is one reason astronomers use transit observations with spectroscopy, especially for hot, large planets where the signal is strongest.

The radial velocity method sees the star wobble

A planet and star orbit their common centre of mass. The planet does most of the moving, but the star moves too. That motion changes the star’s light through the Doppler effect. When the star moves toward us, its spectral lines shift slightly to shorter wavelengths. When it moves away, they shift to longer wavelengths.

This is the radial velocity method. It measures the star’s speed along our line of sight. For a giant planet like Jupiter orbiting the Sun, the Sun moves at about 12.5 metres per second because of Jupiter’s pull. That is about the speed of a slow bicycle. Earth makes the Sun wobble by only about 9 centimetres per second. That is far below what most current instruments can measure cleanly.

The first widely successful radial velocity detections found hot Jupiters, planets with masses similar to or larger than Jupiter and orbits of only a few days. A famous case is 51 Pegasi b, whose presence was inferred from a stellar wobble of about 50 metres per second and an orbital period of just 4.23 days. Today, the best spectrographs can reach roughly 1 metre per second under ideal conditions, and a few aim at even better stability. Even so, stellar activity such as spots and magnetic cycles can create signals that mimic planets.

Radial velocity data tell astronomers the planet’s minimum mass, written as  M \sin i. The angle i is the tilt of the orbit. If a planet also transits, astronomers know i closely and can find the true mass. That is one reason a transiting planet with radial velocity follow-up is so useful. The transit gives the radius, and the wobble gives the mass. Together they give density, which helps tell whether a planet is rocky, watery, or gas-rich.

Why the two methods work well together

Each method has strengths that fill in the other method’s gaps. Transit surveys find many planets at once, including some very small ones, but only if the geometry is right. Radial velocity can find planets that do not transit, but the signal gets harder as the planet gets smaller and farther from the star.

A simple example shows the power of using both. Suppose a planet causes a 1 percent dip in a Sun-like star and orbits every 3 days. The transit depth suggests a planet about 1 Jupiter radius in size. If radial velocity shows a wobble of 100 metres per second, the planet is likely several Jupiter masses. That points to a dense gas giant. If the same-sized planet had only a 3 metre per second wobble, it would be much less massive and far puffier.

For smaller planets, the numbers are harder. A planet the size of Earth transiting a Sun-like star causes only an 84 parts-per-million dip. If that planet orbits a faint red dwarf instead, the dip can be much larger because the star is smaller. That is why many recent discoveries have come from small stars. An Earth-sized planet crossing a star half the Sun’s radius blocks four times as much light, about 0.03 percent.

The timing also matters. A planet on a 365-day orbit transits once a year, so astronomers may need years of observations to confirm it. A 10-day planet gives 36 or 37 transits in a year, which is far easier to verify.

What astronomers have learned from these planets

Exoplanet science has moved from simple detection to basic population studies. Astronomers now know that planets are common. Many stars host multiple planets, and compact systems with several planets inside Mercury’s orbit are frequent. Some planets orbit so close to their stars that a year lasts less than a day. Others travel on long orbits that take years or decades to sample fully.

These methods also show that planetary systems can look very different from our own. Hot Jupiters exist, even though no such planet orbits the Sun. Some planets are rocky and larger than Earth, a class often called super-Earths. Some are smaller than Earth, though they are harder to detect. By combining transit and radial velocity results, astronomers can compare planet density with size and estimate the mix of rock, metal, water, and gas.

The field has also moved into atmosphere studies. For some hot Jupiters, astronomers have detected sodium, potassium, water vapour, carbon monoxide, and carbon dioxide in transmission spectra. Those detections need careful analysis, but they show that exoplanets are not just points of light. They are worlds with measurable physical and chemical properties.

The next missions aim for smaller planets and better atmospheres

The next step is to find more Earth-sized planets around bright nearby stars and to study their atmospheres with higher precision. ESA’s PLATO mission, planned for launch in 2026, is designed to find transiting planets around bright Sun-like stars and to measure their sizes accurately. Bright host stars make follow-up with radial velocity easier, which helps determine planet masses.

NASA’s Nancy Grace Roman Space Telescope, planned for launch no later than 2027, is expected to use microlensing to find planets far from their stars, including planets with masses down to roughly Mars or below in some cases. Microlensing is a different technique from transit and radial velocity, but it fills an important gap by finding cold planets that other methods rarely see.

ESA’s ARIEL, planned for launch in 2029, will study the atmospheres of a large sample of exoplanets. Its main goal is not discovery but characterisation. It will observe transiting planets and measure the light passing through or being emitted by their atmospheres, helping astronomers compare many worlds in a consistent way.

On the ground, large telescopes such as the Extremely Large Telescope and the Giant Magellan Telescope, both under construction in the 2020s, are expected to improve radial velocity work and direct planet imaging. Better stability, larger mirrors, and improved instruments should make it easier to detect lower-mass planets and to study nearby systems in detail.

If you want to see how exoplanet searches fit into the wider field, the Astronomy & Astrophysics Masterclass curriculum gives a structured path through the main tools and ideas used by astronomers: /curriculum.