What Is an Exoplanet?

An exoplanet, or extrasolar planet, is a planet that orbits a star outside our Solar System.

The eight planets in our Solar System orbit the Sun. Exoplanets orbit other stars.

Some exoplanets are enormous gas giants larger than Jupiter, while others are small rocky worlds. Some orbit extremely close to their stars, while others take many years to complete an orbit.

Thousands of exoplanets have now been confirmed, showing that planetary systems are common throughout our galaxy.

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Why Are Exoplanets Difficult to Detect?

Stars are extremely bright compared with planets.

Imagine trying to see a small insect flying beside a powerful spotlight from kilometres away. The light from the spotlight would overwhelm the light reflected by the insect.

A similar problem occurs when astronomers search for exoplanets.

Planets are:

• Much smaller than stars.
• Much dimmer than stars.
• Extremely far from Earth.
• Usually very close to their stars when viewed from Earth.

As a result, astronomers usually do not detect exoplanets by simply photographing them.

Instead, they often detect the effects that planets have on their stars.

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Methods of Detecting Exoplanets

Several techniques are used to discover exoplanets.

Important methods include:

• Transit method
• Radial velocity method
• Direct imaging
• Gravitational microlensing
• Astrometry

The transit and radial velocity methods have been particularly important in discovering and studying exoplanets.

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The Transit Method

The transit method detects a planet when it passes between its star and Earth.

This event is called a transit.

When the planet passes in front of the star, it blocks a small amount of the star's light.

The star therefore appears slightly dimmer.

Astronomers measure the brightness of a star over time.

If its brightness repeatedly decreases by a regular amount, this may indicate that a planet is orbiting the star.

A graph of brightness against time is called a light curve.

A simplified transit light curve might look like:

Normal brightness→dip​→Normal brightness

If the dips occur repeatedly at regular intervals, astronomers can determine the planet's orbital period.

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What Can We Learn from a Transit?

The transit method can provide more information than simply showing that a planet exists.

Planet Size

A larger planet blocks more of its star's light.

Therefore:

Deeper brightness dip → larger planet relative to the star

If astronomers know the size of the star, they can estimate the radius of the planet.

Orbital Period

If a transit occurs every 20 days, the planet takes approximately 20 days to complete one orbit.

Therefore:

Time between transits → orbital period

Atmosphere

A tiny fraction of the star's light can pass through a planet's atmosphere during a transit.

Different gases absorb particular wavelengths of light.

By studying the resulting spectrum, astronomers may be able to investigate gases within an exoplanet's atmosphere.

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Advantages and Limitations of the Transit Method

Advantages

• Can discover large numbers of planets.
• Can determine orbital periods.
• Can estimate planetary radius.
• Can sometimes help scientists study atmospheres.
• Multiple planets can sometimes be detected around the same star.

Limitations

The orbit must have a suitable orientation.

The planet must pass between:

Earth→planet→star

If the orbit is tilted differently from our viewpoint, the planet will not transit its star as seen from Earth.

Therefore, many planets cannot be detected using the transit method from our line of sight.

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The Radial Velocity Method

A planet does not technically orbit around the exact centre of its star.

Instead, the star and planet both orbit their common centre of mass, called the barycentre.

For a star with a much smaller planet, this point is usually inside the star.

As the planet orbits, its gravity causes the star to make a small repeating motion.

Astronomers sometimes describe this as a stellar wobble.

We usually cannot directly see this tiny motion.

Instead, astronomers detect it by examining the star's spectrum.

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The Doppler Effect

The radial velocity method relies on the Doppler effect.

When the star moves toward Earth, its spectral lines shift slightly toward shorter wavelengths.

This is called a blueshift.

When the star moves away from Earth, its spectral lines shift toward longer wavelengths.

This is called a redshift.

Therefore:

Star moving toward us → blueshift

Star moving away from us → redshift

If this pattern repeats regularly, it may indicate that an orbiting planet is gravitationally pulling on the star.

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What Can We Learn from Radial Velocity?

The radial velocity method can provide information about:

• The planet's orbital period.
• The shape of its orbit.
• A minimum estimate of its mass.

A more massive planet generally produces a stronger gravitational effect on its star.

Therefore:

More massive planet → larger stellar wobble → larger radial velocity signal

This makes large planets relatively easier to detect using this method.

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Combining Transit and Radial Velocity Data

The transit and radial velocity methods are especially powerful when used together.

The transit method can provide the planet's:

Radius

The radial velocity method can provide information about its:

Mass

If scientists know both mass and radius, they can estimate the planet's average density:

\( \rho = \frac{m}{V} \)

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Density provides clues about what the planet may be made of.

For example:

• High density may indicate a rocky planet.
• Lower density may indicate a planet containing large amounts of gas.

Combining detection techniques therefore allows astronomers to learn much more about an exoplanet than either method alone.

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Direct Imaging

In some situations, astronomers can actually obtain an image of an exoplanet.

This is called direct imaging.

Special instruments can block or reduce the overwhelming light from the star, making the much fainter planet easier to detect.

Direct imaging works best for planets that are:

• Large.
• Hot and relatively bright.
• Far from their host stars.
• In relatively nearby planetary systems.

Direct imaging is much more difficult for small Earth-like planets close to their stars.

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Gravitational Microlensing

Another technique uses gravity itself to detect planets.

According to general relativity, the gravity of a massive object can bend light.

If one star passes almost directly in front of another distant star, the gravity of the nearer star can temporarily magnify the light of the background star.

This is called gravitational microlensing.

If the foreground star has a planet, the planet can produce an additional small change in the magnification.

This can reveal the planet's presence.

Microlensing can detect planets that may be difficult to discover using other techniques, but the alignment is temporary and usually does not repeat.

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Astrometry

Astrometry involves extremely precise measurements of a star's position in the sky.

An orbiting planet can cause its star to move slightly back and forth relative to more distant stars.

This is another consequence of the star and planet orbiting their common centre of mass.

Astrometry can therefore reveal planets by measuring the star's tiny positional motion.

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Comparing Exoplanet Detection Methods

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Detection Bias

Our catalogue of known exoplanets does not necessarily represent all planets in the galaxy equally.

Some planets are simply easier to detect.

For example, transit surveys are more likely to detect planets that:

• Orbit close to their stars.
• Have short orbital periods.
• Are relatively large.

Radial velocity surveys are particularly sensitive to planets that produce stronger gravitational effects on their stars.

This creates a detection bias.

Scientists must consider these biases when using known exoplanets to estimate how common different types of planets really are.

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Space Telescopes and Exoplanets

Several space missions have transformed our understanding of exoplanets.

The Kepler Space Telescope used the transit method to monitor the brightness of large numbers of stars. It demonstrated that planets are extremely common throughout the Milky Way.

The Transiting Exoplanet Survey Satellite searches for transiting planets around relatively nearby bright stars.

The James Webb Space Telescope can investigate the atmospheres of some known exoplanets using spectroscopy.

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Worked Example: Reading a Transit

Suppose astronomers observe a star for several weeks.

They record significant dips in brightness on:

• Day 4
• Day 14
• Day 24
• Day 34

What can they conclude?

The interval between each transit is:

14 − 4 = 10 days

24 − 14 = 10 days

The pattern repeats every 10 days.

Therefore, the suspected planet has an orbital period of approximately:

10 days​

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Example: Comparing Two Transit Signals

Two planets transit the same star.

Planet A causes a 1% decrease in brightness.

Planet B causes a 4% decrease in brightness.

Which planet is larger?

Planet B blocks more of the star's light.

Therefore:

Planet B has the larger radius.​

In fact, transit depth is approximately related to the relative radii by

\( \frac{ \Delta F }{F} = ( \frac{R_p}{R_s})^2 \)

where:

• R_p​ = radius of the planet
• R_s​ = radius of the star

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Did You Know?

The first confirmed exoplanets were discovered orbiting a pulsar in the early 1990s rather than a normal Sun-like star.

Since then, astronomers have discovered an extraordinary variety of planetary systems, including giant planets extremely close to their stars, planets orbiting two stars, and systems containing several planets packed into relatively small regions.

These discoveries have shown that our Solar System represents only one of many possible planetary arrangements.

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Key Terms

• Exoplanet – a planet orbiting a star outside our Solar System.
• Transit – the passage of a planet in front of its star from our viewpoint.
• Light curve – a graph showing how an object's brightness changes over time.
• Radial velocity – motion toward or away from an observer.
• Doppler effect – change in observed wavelength caused by relative motion.
• Redshift – shift toward longer wavelengths.
• Blueshift – shift toward shorter wavelengths.
• Barycentre – the common centre of mass around which two objects orbit.
• Direct imaging – detecting light from a planet itself.
• Gravitational microlensing – temporary magnification of light caused by gravitational bending.
• Astrometry – precise measurement of the positions and motions of stars.
• Detection bias – the tendency of a detection method to find some types of objects more easily than others.

Key Takeaways

• An exoplanet is a planet orbiting a star outside our Solar System.
• Exoplanets are difficult to observe directly because their stars are much brighter.
• The transit method detects small, repeated decreases in a star's brightness.
• Transit observations can reveal a planet's radius and orbital period.
• The radial velocity method detects the gravitational effect of a planet on its star using Doppler shifts.
• Radial velocity measurements provide information about a planet's mass and orbit.
• Combining transit and radial velocity measurements allows scientists to estimate a planet's density.
• Other detection techniques include direct imaging, gravitational microlensing, and astrometry.
• Each detection method has advantages, limitations, and detection biases.
• Using several techniques together gives astronomers a much more complete picture of planetary systems beyond our own.