Exoplanets: What Backyard Astronomers Can Observe

Here's something that would have seemed impossible a generation ago: you can detect a planet orbiting another star using a telescope that fits in your backyard. You won't see the planet itself, it's far too small and faint, but you can measure the tiny dip in starlight that occurs when the planet passes in front of its host star. This technique, called transit photometry, is the same method that professional space telescopes like Kepler and TESS use, and amateurs are doing it successfully all over the world.

How Transit Detection Works

When a planet passes between its host star and Earth (a "transit"), it blocks a small fraction of the star's light. For a hot Jupiter, a gas giant orbiting very close to its star, the light dip is typically 1-3%. For smaller planets, the dip is less than 0.1% and requires professional equipment. But that 1-3% signal from hot Jupiters is well within reach of amateur gear.

You don't photograph the planet or even see the dip in real time. Instead, you take a continuous series of images of the host star over several hours, measure the star's brightness in each image relative to comparison stars in the same field, and plot a light curve. If a transit occurs during your observation window, the light curve shows a characteristic flat-bottomed dip lasting 1-3 hours.

Equipment You Need

The good news: you don't need exotic equipment. The requirements are modest but specific:

• Telescope: 8-inch (200mm) or larger gives the best results, but transits of the deepest hot Jupiters have been detected with telescopes as small as 4 inches. What matters most is a stable, well-guided mount.
• Camera: A monochrome CCD or CMOS camera is ideal. Color cameras work but with reduced precision. The camera needs low read noise and the ability to shoot long sequences without interruption.
• Mount: A motorized equatorial mount with autoguiding. The telescope needs to track the target star continuously for 3-5 hours without significant drift.
• Filter: A clear or R-band (red) filter helps reduce atmospheric effects. Some observers use no filter at all for maximum signal.

Step-by-Step: Your First Transit Observation

1. Choose a Target

Start with a deep transit, a hot Jupiter with a brightness dip of at least 1% around a star brighter than magnitude +11. Good beginner targets include:

• HAT-P-32b: 1.8% depth, magnitude +11.3 host star, 3.2-hour transit duration.
• WASP-10b: 2.7% depth, magnitude +12.7 host star, 2.2-hour transit.
• HD 189733b: 2.4% depth, magnitude +7.7 host star, one of the easiest targets, very bright.
• TrES-3b: 2.6% depth, magnitude +12.4 host star, 1.3-hour transit.

Use the Exoplanet Transit Database (ETD) or NASA's Exoplanet Archive to find predicted transit times for your location. You need to know exactly when the transit starts and ends to plan your observing session.

2. Plan the Observation

You need data from before, during, and after the transit, ideally starting 1 hour before ingress and continuing 1 hour after egress. This baseline data is critical for establishing the star's normal brightness level.

3. Capture the Data

Set up your telescope and camera, frame the target star and several comparison stars of similar brightness, and start a continuous sequence of exposures. Typical settings:

• Exposure time: 30-120 seconds, chosen to keep the target star well-exposed but not saturated.
• Cadence: Expose continuously with no gaps. A 60-second exposure every ~65 seconds (including readout) is typical.
• Defocus slightly: Spreading the star's light over more pixels reduces the impact of pixel-to-pixel sensitivity variations and makes the photometry more precise. This is counterintuitive but well-established.

4. Process the Light Curve

After the observation, use differential photometry software to measure your target star's brightness relative to comparison stars in each frame:

• AstroImageJ: A free tool specifically designed for exoplanet transit analysis. It handles image calibration, aperture photometry, and light curve fitting.
• HOPS: Another free tool tailored for transit photometry.
• Manual approach: Stack-calibrate images normally, then use aperture photometry in any astro software to extract brightness measurements.

If the transit was captured successfully, your light curve will show the star's brightness holding steady, then dipping by the predicted amount for the predicted duration, then returning to normal. That dip is the planet.

Contributing Your Data

Your transit observations have scientific value. You can submit them to:

• Exoplanet Transit Database (ETD): A community database where amateurs upload transit light curves. Researchers use this data to refine orbital parameters.
• AAVSO Exoplanet Section: The American Association of Variable Star Observers coordinates professional-amateur collaborations in exoplanet science.
• NASA Exoplanet Watch: A citizen science program that actively solicits amateur transit observations to confirm and characterize TESS discoveries.

The more data points astronomers have for each planet, the more precisely they can determine its orbital period, size, and whether additional unseen planets are perturbing its orbit. Your observation becomes part of that dataset.

What You Can't Observe (Yet)

To keep expectations realistic: amateur equipment can detect hot Jupiter transits reliably. Smaller planets (Neptunes, super-Earths) produce transits too shallow for typical amateur setups. Direct imaging of exoplanets requires adaptive optics and coronagraphs far beyond amateur budgets. And spectroscopic detection (measuring wobbles in the star's spectrum) needs professional-grade spectrographs.

But hot Jupiters alone offer dozens of accessible targets, and the field is advancing rapidly. What amateurs can do today was impossible even 15 years ago.