No one has drilled into Jupiter or breathed the air of a planet around another star. Yet astronomers know a surprising amount about what planets and their atmospheres are made of, by combining several methods.
Density: rock, ice or gas?
If you know a planet’s mass (from the orbits of its moons, or how it tugs on its star) and its radius, you can calculate its average density:
- Earth, about 5.5 grams per cubic centimeter: rock and iron.
- Saturn, about 0.7: mostly hydrogen and helium, less dense than water.
- In between: water-rich or ice-rich worlds.
For exoplanets, the transit method gives the radius and the radial velocity method gives the mass, see the methods used to find exoplanets.
Spectroscopy
Molecules absorb and emit light at specific wavelengths. By splitting light from a planet into a spectrum, astronomers identify gases such as water vapor, methane, carbon dioxide and ammonia, see what starlight reveals.
- Reflected light: sunlight reflected by a planet carries the fingerprints of its atmosphere and clouds.
- Transmission spectroscopy: when an exoplanet crosses its star, a little starlight filters through its atmosphere. In 2022, the James Webb Space Telescope made the first clear detection of carbon dioxide in an exoplanet atmosphere, on the hot gas giant WASP-39b, see what JWST does.
- Thermal emission: infrared light reveals temperatures and sometimes day-night differences.
Visiting the planets
Within the solar system, spacecraft measure directly:
- Probes dropped into atmospheres: the Galileo probe into Jupiter (1995), Huygens onto Titan (2005), Soviet Venera landers on Venus.
- Rovers and landers: Curiosity and Perseverance analyze Martian rocks and air; Perseverance collects samples for possible return to Earth.
- Seismology: NASA’s InSight lander detected marsquakes that revealed the size of Mars’s core.
- Gravity mapping: Juno measures Jupiter’s gravity field to probe its interior.
Clues from meteorites
Some meteorites come from Mars, the Moon and asteroids. Lab analysis of these rocks, and of samples returned by missions such as Apollo, Hayabusa2 and OSIRIS-REx, shows the building blocks of planets, see comets and asteroids.
What these methods have revealed
The rocky inner planets have thin or no atmospheres, except Venus with its crushing carbon dioxide atmosphere. The giants are mostly hydrogen and helium. Many exoplanets have no equivalent in our solar system, such as “mini-Neptunes” with thick atmospheres. Planets’ magnetic fields offer more clues about their interiors, see planetary magnetic fields.
