L29. Exoplanets: Worlds Around Other Stars

Unit 6 · Galaxies and the Universe

R-report

L29. Exoplanets: Worlds Around Other Stars

Before 1995, nobody had confirmed a single planet around another ordinary star. We now know of more than five thousand, including some roughly Earth-sized in the right place for liquid water.

Finding planets you cannot see

Photographing a planet around another star is extraordinarily hard. The planet is tiny, dark, and sits right beside a star billions of times brighter. So almost all exoplanets are found indirectly, by their effect on the star. The transit method is the most productive. If a planet's orbit happens to pass between us and its star, the star dims very slightly, then returns to normal. Measure that dip repeatedly and you learn a great deal. - The depth of the dip gives the planet's size relative to the star. - The time between dips gives the orbital period, and therefore the distance from the star. - The dips are small: Earth crossing the Sun would dim it by about 0.008%. The radial velocity method is the other major technique. A planet's gravity makes its star wobble slightly, and that wobble shows up as a tiny back-and-forth Doppler shift in the star's spectrum. This gives the planet's mass. Use both on the same planet and you get size and mass — and therefore density, which tells you whether it is rocky or gaseous.

A stranger variety than expected

The first discoveries overturned expectations immediately. Astronomers had assumed other systems would resemble ours: small rocky planets close in, giants far out. Instead the first planet found around a Sun-like star was a gas giant orbiting closer than Mercury, completing a year in four days. These "hot Jupiters" were not predicted. Other surprises followed. - Super-Earths, larger than Earth but smaller than Neptune, are extremely common — and there are none in our solar system. - Many planets orbit in strongly elliptical paths rather than near-circles. - Planets exist around binary stars, orbiting both. - Some planets appear to have been ejected and now drift alone through space. Statistically, planets appear to be the rule rather than the exception. Current estimates suggest most stars in the galaxy host at least one planet, which means the Milky Way contains hundreds of billions of them.

The habitable zone, and its limits

The habitable zone is the range of distances from a star where a planet could hold liquid water on its surface — not so close that water boils away, not so far that it freezes. Where that zone sits depends on the star. A cool red dwarf has a narrow zone very close in; a hot bright star has a wide zone much farther out. But being in the zone is only a starting condition, not a guarantee. Several other factors matter. - Size: too small and the planet cannot hold an atmosphere; too large and it becomes a gas giant. - Atmosphere: Venus sits near the edge of the Sun's habitable zone and is a furnace because of its atmosphere. - Stability: red dwarfs often emit violent flares that could strip a nearby planet's atmosphere. - A magnetic field helps protect an atmosphere from being blown away by stellar wind. The next step is measuring exoplanet atmospheres. When a planet transits, a little starlight filters through its atmosphere and picks up absorption lines from the gases there. Space telescopes are now beginning to read those spectra — the most direct route we have toward answering whether any of these worlds resembles ours.

Putting the investigation together

Exoplanets are found mainly through the tiny effects they have on their stars: dips in brightness during transits and Doppler wobbles from gravitational tugs. Combining both gives size, mass, and density, which is enough to distinguish a rocky world from a gas giant.

The scientific value of the field goes beyond counting planets. Discovering hot Jupiters and super-Earths forced theorists to rebuild models of how planetary systems form, models that had been based on a sample size of one. Being wrong in an interesting way is often more productive than being right about too little data.