So far in this module, you’ve seen how the first exoplanets were discovered around a pulsar and then around a Sun-like star (51 Pegasi). In this section, you’ll explore the main indirect techniques astronomers use to detect planets that are too faint and too close to their stars to see directly.
All three methods you’ll study here rely on the same basic idea: even when you can’t see a planet itself, you can detect the effects it has on light from its star or from a more distant background object.
Radial Velocity Method
The method used to discover 51 Pegasi b is known as the radial velocity method. When a planet orbits a star, the two bodies actually orbit their common centre of mass, causing the star to “wobble” slightly. As the star moves toward and away from us, its light is Doppler shifted: spectral lines shift to shorter wavelengths (blueshift) as it approaches and to longer wavelengths (redshift) as it recedes.
By carefully measuring these tiny shifts over time, astronomers can determine the star’s radial velocity (its motion along our line of sight) and infer the presence of an orbiting planet.
The radial velocity signal is stronger for:
- More massive planets (they tug harder on the star), and
- Planets closer to the star (gravity is stronger at smaller distances).
From the radial velocity curve, astronomers can estimate:
- the planet’s orbital period (from the timing of the wobble), and
- a minimum value of the planet’s mass (often written as M sin i, where i is the tilt of the orbit).
This method is still one of the most important tools for measuring exoplanet masses. It has been used to detect everything from hot Jupiters to nearby, potentially rocky worlds like Proxima Centauri b.
Transit Method
While radial velocity detects a planet’s tug on its star, the transit method detects a planet by watching for a tiny dip in a star’s brightness when the planet passes (or transits) in front of it.
For a transit to be visible, the planet’s orbit must be aligned so that it crosses the star from our point of view. This is relatively rare, but when it happens the star’s light curve shows a repeatable dip whose depth is set by the ratio of the planet’s area to the star’s area.
From a transit light curve, astronomers can determine:
- the planet’s orbital period (from the spacing of repeated dips), and
- the planet’s radius (from how much starlight is blocked).
If the same planet is also detected via radial velocity, combining the two methods gives both mass and radius, and therefore the planet’s average density. This helps distinguish between gas giants, ice giants, rocky “super-Earths,” and lower-density mini-Neptunes.
The Kepler space telescope and its extended K2 mission used the transit method to monitor over 150,000 stars, revealing thousands of exoplanet candidates and showing that small planets are common. Today, NASA’s TESS mission continues this work across the whole sky, while ESA’s upcoming PLATO mission is designed to find and characterize Earth-sized planets around bright nearby stars.
Microlensing Method
The third major indirect technique is the microlensing method, which relies on gravitational lensing — a prediction of general relativity. Massive objects bend spacetime, causing light from more distant sources to be magnified and distorted as it passes by.
On large scales, gravitational lensing produces dramatic arcs and rings around massive galaxy clusters, as in the “Cheshire Cat” galaxy cluster:
On smaller scales, similar lensing events occur when one star passes almost exactly in front of another from our point of view. The foreground star briefly brightens the background star’s light in a symmetric “bell-shaped” curve. This is called a microlensing event.
If the foreground star has a planet, the planet’s gravity can produce an extra, shorter brightening within the main lensing event. By modelling this anomaly, astronomers can measure the planet’s mass and its separation from the host star.
Microlensing has some distinctive strengths:
- It is most sensitive to planets a few astronomical units from their stars — roughly the region of Jupiter and Saturn in our Solar System.
- It can detect planets around faint or distant stars on the far side of the Galaxy.
- It can even reveal rogue planets that do not orbit any star at all.
Ground-based surveys such as OGLE and MOA have discovered dozens of planets using microlensing. In the future, NASA’s Roman Space Telescope is expected to find thousands more, giving a powerful statistical picture of planets in the outer regions of planetary systems.
Learning Activity: Comparing Indirect Detection Methods
As you review the three methods above, think about how they complement each other. In your own words, answer the following questions:
- For each method (radial velocity, transit, microlensing), what property or effect of the planet or star is being measured?
- Which method is best suited to detecting:
- Close-in hot Jupiters?
- Small, Earth-sized planets in short-period orbits?
- Planets far from their stars or even rogue planets?
- Which combinations of methods can give both the planet’s radius and mass, and therefore its density?
- What are some observational limitations or biases of each method (for example, the need for particular orbital alignments or very precise instruments)?
Use a few bullet points to summarize how these methods, taken together, give us a much more complete picture of planetary systems than any one method alone.
To see how these methods have transformed our understanding over time, examine the following plot from the NASA Exoplanet Archive. It shows the cumulative number of confirmed exoplanets as a function of discovery year. Notice how the discovery rate accelerates after the launch of Kepler (2009) and again after TESS (2018):
In the next section, you’ll look at direct methods—ways of actually separating a planet’s light from its star and tracking the star’s motion on the sky—to push beyond detection and toward detailed characterization of distant worlds.
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