Why planets are hard to see
A planet shines only by reflecting its star’s light, and a star is enormously brighter than any planet near it. Seen from far away, a planet’s faint glow is lost in the glare, a bit like trying to spot a firefly next to a searchlight. For this reason, most exoplanets, the planets beyond our Solar System, have been found indirectly, by looking for the effects a planet has on its star.
The first confirmed planet around a Sun-like star, 51 Pegasi b, was announced in 1995. Since then, the number of confirmed exoplanets has grown into the thousands.
The transit method
If a planet’s orbit happens to be edge-on from our point of view, the planet passes in front of its star once per orbit. During this transit, a small fraction of the starlight is blocked, and the star dims very slightly and briefly. Telescopes that monitor the brightness of many stars can look for these repeating dips.
The size of the dip tells us about the planet’s size. The dimming roughly equals the ratio of the planet’s disk area to the star’s disk area. As a worked example with rounded numbers, Jupiter’s radius is about one tenth of the Sun’s, so a Jupiter-sized planet passing in front of a Sun-like star would block about one percent of the light (one tenth squared is one hundredth). Earth’s radius is about one one-hundred-and-tenth of the Sun’s, so an Earth-sized planet would block less than a hundredth of a percent. That is a tiny change, which is why finding Earth-sized planets requires very precise instruments, often in space.
The time between dips gives the orbital period, and from this and the star’s mass, astronomers can estimate how far the planet is from its star. Space missions such as Kepler and TESS were designed around this method.
A limitation is geometry. Only a small fraction of planetary systems are aligned so that planets transit, so many planets are missed. Also, a dip alone is not always enough; astronomers usually confirm candidates with additional observations to rule out look-alikes, such as a pair of stars eclipsing one another.
The wobble method
A star does not stay perfectly still when a planet orbits it. Both the planet and the star orbit their shared center of mass. Because the star is much heavier, its motion is small, but it can be detected as a slight back-and-forth wobble.
Astronomers measure this wobble using the Doppler effect. When the star moves toward us, its light shifts slightly to bluer wavelengths; when it moves away, to redder ones. By spreading starlight into a detailed spectrum, much like the way the visible spectrum of white light spreads into colors, astronomers can measure tiny periodic shifts in the spectral lines. This is often called the radial velocity method.
The size of the wobble relates to the planet’s mass. A massive planet close to its star produces a bigger signal than a small planet far away. As a rough scale, Jupiter tugs the Sun at a speed on the order of ten meters per second, while Earth does so at only a few centimeters per second. This shows why early discoveries tended to be giant planets and why finding Earth-like worlds this way is demanding.
Transits and wobble complement each other. Transits give a radius, wobble gives a minimum mass, and if a planet shows both, astronomers can estimate its density and judge whether it is more likely rocky or gaseous.
Other methods
- Direct imaging. In favorable cases, astronomers block most of a star’s light and capture a planet directly. This works best for young, large, hot planets far from their stars, where the planet is still glowing from its formation.
- Gravitational microlensing. When a star passes in front of a more distant star, its gravity briefly magnifies the background light. A planet orbiting the foreground star can add a short extra blip. This method can detect planets far from their stars, but each event happens only once.
- Astrometry. This measures the tiny sideways shift in a star’s position on the sky caused by orbiting planets. It requires extremely precise position measurements.
- Timing methods. Regular signals, such as the pulses from a pulsar, can show small timing shifts if planets are present. The first confirmed planets were found around a pulsar this way, a few years before 51 Pegasi b.
What the detections have revealed
Detection methods favor certain planets, so the catalog is not a fair sample. Large planets on short orbits are easiest to find, which is why early surveys turned up many “hot Jupiters,” giant planets orbiting very near their stars. Statistical studies that correct for these biases suggest that planets are common, and that small planets are numerous. Many systems look quite different from our own, with planets of sizes between Earth and Neptune, which have no equivalent among our solar system’s gas giants and rocky worlds.
Astronomers can sometimes also study planetary atmospheres. When a planet transits, a little starlight filters through its atmosphere, and the chemical fingerprints in that light can be read from the spectrum. Such work is difficult and often yields tentative results, so claims should be read carefully.
What this means for the search for life
Finding planets is not the same as finding life. Researchers consider whether a planet is the right size and distance from its star to allow liquid water, but this is only one factor among many. The topic of extraterrestrial life is largely about what future observations might reveal, and for now there is no confirmed detection of life beyond Earth.