Planet Hunters CHEOPS

What is CHEOPS?

CHEOPS (CHaracterising ExOPlanet Satellite) is a European Space Agency mission launched in December 2019. It measures the brightness of stars with extraordinary precision from Earth orbit. Its main goal is to characterise exoplanets that have already been discovered by other surveys, but every observation it takes also captures many other stars in the field of view — and those stars have never been searched for planets in detail. That is where you come in.

What exactly am I looking at?

You are looking at light curves: graphs showing how the brightness of a star changes over time. Each point is a measurement of the star's brightness taken by CHEOPS. When a planet passes in front of its star (a "transit"), it blocks a tiny fraction of the starlight, producing a small, roughly U-shaped or box-shaped dip in the light curve. Your task is to spot those dips, mark where they start and end, and tell us whether you think the signal looks like a potential planet or just noise.

Where does the data come from?

We have extracted light curves from thousands of background stars that appear in more than five years of public observations taken by CHEOPS. These stars were not the intended targets of the observations, so their light curves have never been examined in detail — to our knowledge, nobody has looked at them before you.

Do I need any astronomy background?

No. The tutorial and field guide will show you what transits look like and how to tell them apart from common sources of noise. All you need is a keen eye and a few minutes. Many of the most interesting discoveries in citizen science projects have been made by people with no formal training in astronomy.

Why do you need humans? Can't computers do this?

Computers are excellent at finding regular, obvious, repeating signals, but they struggle with shallow, noisy, single, or unusual events — exactly the kind of signals that may hide small Earth-sized planets, long-period planets, or multi-planet systems. The human eye remains remarkably good at spotting subtle patterns that automated pipelines overlook. Projects like Planet Hunters TESS have proven this by finding confirmed planets that algorithms missed.

What if I'm not sure whether something is a transit?

Mark it anyway and classify it as best you can. Every light curve is reviewed independently by many volunteers, so no single classification decides the outcome. Combining thousands of classifications lets us filter out noise and identify the signals that many people agree on. An honest "best guess" is genuinely useful; skipping something because you are unsure is not.

What happens after I classify a light curve?

Classifications from all volunteers are combined statistically. Light curves where many people mark the same dip are flagged as candidates and passed to the science team for detailed vetting: checking the CHEOPS data for instrumental effects, looking for the same signal in other datasets, and, when warranted, requesting follow-up observations. The strongest candidates can lead to confirmed planet discoveries.

Will I be credited if a planet is found?

Yes. Following the tradition of previous Planet Hunters projects, volunteers whose classifications contribute to a discovery are credited in the resulting peer-reviewed publications, and in past projects volunteers have appeared as co-authors on discovery papers.

How is this different from Planet Hunters TESS?

The approach is the same, but the data are different. TESS surveys large areas of the sky with wide-field cameras, while CHEOPS points at individual stars with very high photometric precision. This project searches the serendipitous background stars captured in those pointed observations — a dataset that has never been systematically explored for transits before.

How much time does it take?

As much or as little as you like. Classifying a single light curve typically takes under a minute once you are familiar with the interface. Every classification counts, whether you do five or five thousand.

I found something strange that doesn't look like a transit. What should I do?

Post it on the project's Talk board! Unusual signals — eclipsing binaries, stellar flares, pulsating stars, or things nobody can immediately explain — are often scientifically interesting in their own right. Several famous discoveries in citizen science began with a volunteer asking "what is this weird thing?" on Talk.

Who is behind this project?

The project is led by scientists at the European Space Agency and at the CHEOPS Science Team working with the CHEOPS mission. You can reach the team on the project's Talk board, where we regularly answer questions and discuss interesting finds with volunteers.