Exoplanets: How Do We Find Planets Around Other Stars?

What is an exoplanet, and why is it hard to see?

An exoplanet (or extrasolar planet) is a planet that goes round a star other than our Sun. More than 5,000 are known today.

Why is it hard to see one? A star is millions of times brighter than its planet, and the planet sits very close to it in the sky. It is like trying to see a firefly next to a huge searchlight, from a city far away. So scientists use indirect methods: they watch the star and look for tiny changes caused by the planet.

The transit method

A transit happens when a planet passes in front of its star as seen from Earth. The planet blocks a little starlight, so the star looks dimmer for a few hours. Then it gets bright again.

A graph of brightness against time is called a light curve. A transit makes a small dip, like a shallow dent, and the dip repeats every time the planet goes round.

What the dip tells us

• Depth of the dip tells the planet's size: dip = (R_planet / R_star)².
• Time between dips is the planet's orbital period (its year).
• Length of the dip tells how fast it moves across the star.

A planet like Jupiter blocks about 1% of a Sun-like star. An Earth-size planet blocks only about 0.008%, so we need space telescopes to see it.

A limit

We only see a transit if the orbit is tilted just right, edge-on to us. Most planets never cross their star as seen from Earth, so the method misses many of them. Telescopes watch many stars at once to catch the lucky ones.

The radial velocity (wobble) method

A planet and its star pull on each other. The star is much heavier, so it moves only a little: it makes a tiny circle around the common centre of mass. We call this a wobble.

When the star moves toward us, its light waves get squeezed, so the light shifts to the blue end. When it moves away, the waves are stretched, so the light shifts to the red end. This is the Doppler effect, the same reason an ambulance siren sounds higher as it comes near.

The speed along our line of sight is the radial velocity. A heavy planet close to its star makes a bigger wobble, so it is easiest to find. The time for one blue-red cycle is the planet's year.

This method gives the planet's mass (at least). Transit gives the size. With both, we find the planet's density and can tell a rocky world from a gas giant.

Other methods and what we learn

• Direct imaging: a mask blocks the star's glare so a big, young, far-out planet can be photographed. It is rare.
• Microlensing: a star's gravity bends the light of a farther star behind it, like a lens. A planet adds a short extra flash. It happens only once, but it finds far-away planets.

Planet year and distance

For a star like the Sun, a planet at distance d (in AU) has a year of T = d^1.5 years. A planet at 4 AU takes 8 years. (This is Kepler's third law.) Hot Jupiters are giant planets very close to their star with years of only a few days.

Try it

Open the 3D. Make the planet bigger and watch the dip get deeper. Move it far away and watch the time between dips get longer.

Key formulas and definitions

• Transit dip = (R planet / R star)^2
• Dip in percent = (R planet / R star)^2 x 100
• Year of a planet round a Sun-like star: T (years) = d^1.5, d in AU
• Time between two transits = orbital period
• Blueshift: star moving toward us. Redshift: star moving away

Worked examples

Common mistakes

• Thinking we see exoplanets directly in photos. Most are found by clues: dips in starlight or wobbles.
• Forgetting to square the size ratio. A planet half the star's size does not block half the light; it blocks one quarter.
• Thinking every planet transits. Only planets whose orbit is edge-on to us cross the star.
• Mixing up blueshift and redshift. Blue means moving toward us; red means moving away.