Transit Method

Thousands of planets are now known to orbit other stars, yet almost none of them have ever been directly photographed. Astronomers instead had to become detectives, learning to spot a planet's fingerprints in the light of its star rather than the planet itself — and the transit method is the most productive of these detective techniques.

Why exoplanets can't simply be photographed

An exoplanet (or extrasolar planet) is a planet orbiting a star other than the Sun. The obvious way to find one might seem to be pointing a sufficiently powerful telescope at a star and looking for a faint dot of reflected light nearby — but in practice this almost never works, for two compounding reasons.

• Enormous brightness contrast. A star generates its own light through nuclear fusion, while a planet only reflects a tiny fraction of the light falling on it. A star can easily outshine even a large planet orbiting it by a factor of a billion or more, so the planet's feeble reflected glow is utterly swamped by the star's glare.
• Vanishingly small angular separation. Because exoplanetary systems lie so far from Earth, the angular separation between a planet and its star, as seen from Earth, is extremely small — often far smaller than a telescope's resolving power can distinguish, even before the brightness problem is considered.

Together these mean direct imaging — literally photographing a planet as a separate point of light from its star — is only possible in a handful of favourable cases, typically very young, very large planets orbiting a long way from a relatively faint star. For the vast majority of known exoplanets, astronomers instead rely on indirect detection methods: techniques that infer a planet's existence, size, mass and orbit from the small, measurable effect it has on the star itself or on the star's light, without ever directly seeing the planet. This module covers two such methods — the transit method, in this lesson, and the radial velocity (Doppler) method, in the next.

The transit method: planets as tiny eclipses

If a planet's orbital plane happens to lie edge-on (or very close to edge-on) as seen from Earth, then once per orbit the planet passes directly in front of the star's disc from our point of view. This event is called a transit. During a transit the planet blocks a small fraction of the star's light from reaching Earth, so the star's measured brightness dips very slightly for the duration of the crossing, then recovers once the planet has moved off the star's disc. Because the planet's orbit is periodic, this dip repeats at every orbit, once per period.

Astronomers detect transits by recording a light curve — a graph of a star's measured brightness against time, built up from a telescope monitoring the star continuously (or at frequent regular intervals) over a long period. A transiting planet leaves an unmistakable, repeating signature in this light curve:

• Brightness stays flat and steady while no transit is in progress.
• Brightness drops relatively sharply as the planet begins to cross in front of the star's disc (ingress).
• Brightness stays flat again, but at a slightly lower level, while the planet is fully silhouetted against the star (mid-transit).
• Brightness rises sharply back to its normal level as the planet finishes crossing the disc (egress).

Two features of this light curve carry the physics. The depth of the dip — how much the brightness falls — depends on how much of the star's disc the planet covers, which depends on the ratio of the planet's cross-sectional area to the star's. The spacing between successive dips gives the planet's orbital period directly, since the planet returns to transit position once every orbit.

The key limitation: geometry has to cooperate

The transit method's biggest weakness is built into its basic requirement: it only works for systems whose orbital plane happens to be aligned very close to edge-on as seen from Earth. For any given exoplanetary system, the orientation of the planet's orbit relative to Earth is essentially random, so only a modest fraction of systems will happen to have the right alignment to show transits at all. This means the transit method inevitably misses the majority of exoplanetary systems out there — not because those planets don't exist, but simply because their orbits don't happen to line up with our line of sight. A star with an undetected non-transiting planet is not evidence of an empty system; it may simply be a system viewed from the "wrong" angle.

Despite this limitation, the transit method has been extraordinarily productive, because it can be applied automatically and simultaneously to enormous numbers of stars: a telescope can monitor the brightness of thousands of stars in the same field of view at once, watching for the tell-tale repeating dips, without needing to target any single star in advance.