Topics Space and the universe

What are exoplanets?

Exoplanets are planets that orbit stars other than our Sun. The first ones around Sun-like stars were confirmed in the 1990s, and since then astronomers have confirmed thousands of them. Most stars, it turns out, seem to have at least one planet, which means our solar system is one arrangement among a great many.

What makes them interesting is how little we can see. Almost every exoplanet is far too faint and too close to its star to photograph directly, so astronomers find them from tiny clues: a star that dims slightly as a planet crosses in front of it, or a star that wobbles under a planet's pull. From those clues they work out size, mass, orbit and sometimes the makeup of the air. The planets they have found include worlds nothing like ours, such as gas giants hugging their stars and rocky planets bigger than Earth.

An episode on bre would walk through how these planets are found, what kinds exist, and why the habitable zone is a starting point rather than a promise of life. The hosts are AI, so they can make mistakes, and you can press Talk to ask a question of your own while you listen.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. What counts as an exoplanetA planet outside our solar system, orbiting another star. We start with what makes something a planet and why the first confirmations in the 1990s were a big deal.
  2. Finding planets you cannot seeThe transit method watches for a star dimming as a planet passes in front. The wobble method looks for a star tugged back and forth. Both read faint clues rather than pictures.
  3. The zoo of worldsHot gas giants, rocky planets larger than Earth, and worlds between the size of Earth and Neptune. Many of these have no counterpart in our solar system.
  4. The habitable zone, explainedThe range of distances where liquid water could in principle exist on a surface. Being in it says nothing certain about whether a planet is actually livable.
  5. Reading the air of a distant worldWhen a planet crosses its star, some starlight passes through its atmosphere. Telescopes can look for the fingerprints of gases in that light, though this is hard and still limited.
  6. What we still do not knowWhether any exoplanet has life, how common Earth-like planets are, and where the limits of current methods lie. We say plainly what is open.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Cal. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    Okay, start simple. A planet around another star. We've never seen one up close, never visited one. So how do we even know they're there?

  2. CalAI host

    Short answer: shadows. Longer answer: a star is absurdly bright, and a planet next to it is like a firefly beside a stadium floodlight. So you stop looking for the firefly and watch the floodlight.

  3. breAI host

    Watch it do what?

  4. CalAI host

    Dim. If a planet crosses in front of its star, the star gets a little fainter, then recovers. Repeat on a schedule and you've got an orbit. Hold on, how does that actually work for finding size?

  5. breAI host

    Good question, because dimming sounds crude. A bigger planet blocks more light, so the dip tells you roughly how wide it is.

  6. CalAI host

    Right, and that's the wild part. From a tiny flicker you get a size and a year length. I'd be bad at this job, I'd blink and miss it.

  7. breAI host

    Which is why we use telescopes and not blinking. But there's a second trick, and it works on stars that wobble.

  8. CalAI host

    Yes. A planet tugs on its star, so the star shifts a little too. Different clue, same story: something is out there.

Questions people also ask

How many exoplanets have been found?
Thousands have been confirmed, and the count keeps growing as new data is checked. The exact total changes often, so it is best to look at a current catalog such as NASA's. Most were found by watching stars dim as planets pass in front of them.
Can we see exoplanets directly?
Rarely. Most are found indirectly, through changes in their star's light or motion. A small number of young, large planets far from their stars have been photographed directly, using instruments that block the star's glare. Images of small, Earth-like planets are still beyond current technology.
Are there Earth-like exoplanets?
Some rocky planets of roughly Earth's size have been found in the habitable zone of their stars. But size and distance alone do not tell us whether a planet is truly like Earth. Atmosphere, water and surface conditions are mostly unknown, so astronomers are cautious.
Do exoplanets have life?
We do not know. No confirmed sign of life has been found on any exoplanet. Scientists look at atmospheres for gases that could be linked to biology, but such signs would be hard to interpret and would need careful, repeated checking.

Related topics

More: all 300 topics, space and the universe, or the longer reads on /learn.

bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.