How a Transit Reveals an Exoplanet’s Size

An exoplanet’s size is determined by how much its star dims when the planet passes in front of the star’s disk.

During a transit, the planet blocks some of the starlight. The deeper the dip in brightness, the larger the fraction of the star’s disk the planet covers; to estimate the planet’s radius, astronomers also need to know the star’s radius. In this way, a small change in brightness becomes a way to measure a world that cannot be seen directly.

• about 48 light-years distance to LHS 1140 b

What exactly is measured during a transit?

During a transit, astronomers measure how much a star’s brightness drops as a planet passes in front of its disk from the observer’s point of view. A telescope records this brief dip in light.

Kepler searched for such dips, which recur at regular intervals as a planet passes in front of its star. Repetition helps distinguish a transit signal from a one-off change in brightness.

How a dip in brightness relates to size

The depth of the dip depends on the ratio of the planet’s disk area to the star’s disk area. All else being equal, a larger planet blocks more of the star’s disk, so the light dims more.

But the same drop in brightness does not mean planets are the same size: the result also depends on the star’s radius. If stars differ in size, planets that block the same fraction of their disks may also differ in size. That is why the star’s radius must be taken into account when estimating a planet’s radius.

What does transit depth tell us?

Transit depth shows how much a star’s light dims and allows astronomers to estimate the ratio of the planet’s radius to the star’s radius, but not the planet’s absolute size directly.

On a light curve—a graph showing changes in a star’s brightness—a transit appears as a dip. Its depth depends on what fraction of the star’s visible surface the planet blocks. To determine the planet’s radius, astronomers need to know the star’s radius, established through separate observations.

The Kepler space telescope discovered the Earth-sized exoplanet HD 137010 b orbiting a bright, Sun-like star. This example shows why a dip in a light curve cannot be read as a direct measure of the planet: its depth is also related to the star’s size.

That is why the estimated radius of HD 137010 b should be considered alongside the properties of its star. If the star’s radius is determined differently, the conversion of the measured ratio into the planet’s radius will change too.

Why doesn’t every planet transit?

A transit is visible only when the planet’s orbit is oriented so that it passes in front of its star from the telescope’s point of view; with a different geometry, the star is not obscured.

Even with a suitable orbital inclination, the signal can easily be missed: observations may not coincide with the brief transit, or the dimming may be too slight to detect reliably. Repeated dips in brightness help distinguish a transit from a one-off change in the star, but a planet on a distant orbit requires long periods of observation between transits.

Why orbital geometry matters

LHS 1140 b is about 48 light-years away and orbits in the habitable zone of a red dwarf. To measure its size using the transit method, a telescope must see the planet pass across the star’s disk: being in the habitable zone does not guarantee that its orbit is oriented for such an observation.

When is one transit not enough?

One transit is not enough to determine an exoplanet’s mass and density: it primarily allows astronomers to determine its radius. To find its mass, astronomers measure how the planet affects the star’s motion, then use the mass and radius to estimate its density.

A transit also reveals an atmosphere

During a transit, some of the star’s light passes through the planet’s atmosphere. Atmospheric substances leave spectral signatures that can be used to study its composition. Observations by the James Webb Space Telescope of the hot planet GJ 486 b revealed signs of water vapor.

On the gas giant WASP-94A b, scientists found that one side is cooler and cloudier, while the other is hotter and clearer. Such observations show that a transit is useful for more than determining a planet’s size: the spectrum of its light can reveal differences in its atmosphere.

When does the usual explanation of a transit fail?

The usual explanation of a transit fails if a dip in brightness is taken as proof of a planet—or the absence of a dip as proof that no planet is there.

A planetary signal is checked through repeated observations and compared with the star’s properties. A single dimming event may be a transit candidate, but it needs to be confirmed. A planet’s size is estimated from the depth of the dip together with the star’s parameters: without them, the shape of the signal alone does not provide a reliable size.

The transit method detects a planet only when, from our point of view, it passes in front of the star’s disk. If its orbit is tilted differently, the planet may exist, but its transit will not be observable.

What to check for a candidate

• Transit depth: the measured dimming, not standalone proof that a planet is present.
• The star: its properties are needed to estimate the planet’s size from the depth of the dip.
• Additional observations: repeated signals and other checks help determine whether the dip is really caused by a planet.
• Spectral signatures: clouds and atmospheric properties affect them; a single transit cannot establish that a planet is habitable.

When describing a candidate, these quantities should be considered separately: transit depth, the star’s parameters, and the results of additional observations.

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