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Until the 1990s, the only planets we knew existed were in orbit around the Sun. Since 1992, astronomers have discovered thousands of exoplanets: worlds in orbit around other stars. Based on the data we have, researchers think there could be hundreds of billions of planets in the Milky Way alone.

Current exoplanet research takes many forms: developing methods to find new worlds, looking for signs of certain molecules in their atmospheres, and studying newborn planets around young stars. Astronomers are working on the next generation of telescopes to find Earth-like worlds, and possibly signs that life might exist on other planets.

A wealth of planets … and another Earth?

Astronomers first discovered exoplanets orbiting neutron stars in 1992, followed by the first exoplanets in orbit around ordinary stars in 1995. Those first worlds were much larger than Jupiter, and detected by the way their gravity tugged back on their host stars.

Most exoplanets found since then were discovered by the tiny amount of light they block when they pass between Earth and their host stars. This miniature eclipse is called a “transit”. The duration of the transit and how much light gets blocked tells astronomers the size of the exoplanet and how long it takes to orbit its host star.

Since large planets orbiting close to their stars block more light, most exoplanets we know of are larger than Earth. In fact, the most common exoplanet types identified so far are super-Earths, which are probably rocky planets larger in size than Earth, and mini-Neptunes, worlds slightly smaller than Uranus and Neptune.

Since transits allow us to measure the size of exoplanets but not their mass, researchers have to rely on other methods to determine how massive they are, which in turn tells us if they’re rocky or made of compressed gas. These methods involve measuring gravitational effects, so they are biased toward very massive exoplanets. Some next-generation instruments  — such as the GMT-Consortium Large Earth Finder (G-CLEF) for the Giant Magellan Telescope — are designed in part to do this kind of observation, but in the meantime we only have mass information for some of the exoplanets. Based on theoretical models, astronomers think super-Earths are rocky, while mini-Neptunes are made of compressed gases. These types of planets don’t occur in our Solar System, leading researchers to question how they might be different from worlds we know.

Additionally, many exoplanet systems are tightly packed, with one or more planets orbiting their star much closer than Mercury orbits the Sun. Astronomers are investigating models for planet formation and evolution that would explain both why these systems are common in the galaxy, and why our Solar System doesn’t look like that.

Astronomers have found many exoplanets in their stars’ habitable zones: the range of orbits where liquid water could exist on the planet’s surface, with the proper kind of atmosphere. So far, these discovered worlds are either much larger than Earth or the star is much smaller and redder than our Sun. To find Earth-like worlds orbiting in the habitable zone of Sun-like stars requires long periods of observation, which allows for observing multiple orbits. Even then, these effects are small: the amount of light blocked by a small planet during a transit is even smaller for larger orbits.

Current and upcoming observatories are designed to find even more exoplanets with greater sensitivity. That will let us see more Earth-sized and smaller worlds, along with a larger number of planets orbiting farther from their stars. With this data, we’ll have a better idea of where the Solar System fits into the catalog of all planetary systems, and understand how common worlds like ours might be.

Artist's impression of the exoplanet LHS 1140b, which orbits its star within the "habitable zone" where liquid water might exist on the surface. The LHS 1140 system is only about 40 light-years from Earth, making it a possible target for studying the atmosphere of the planet if it has one.

Credit: M. Weiss/CfA


Breath From Distant Worlds

The sizes and orbits of planets are only part of the picture, which includes their composition. The chemistry both of exoplanets and of their atmospheres is essential for understanding their history, evolution, and — potentially — if they could support life.

So far, no observatory can study the surfaces of exoplanets, but astronomers have begun to study their atmospheres. The way to do this is challenging: when an exoplanet transits its host star, a tiny amount of the light passes through the atmosphere. By analyzing what colors of light are absorbed or emitted by atmospheric gases, astronomers can identify certain molecules like water, oxygen, or methane. It’s a tiny effect on top of an already tiny signal,  like trying to see through and study the antenna of a moth flying in front of a spotlight.

The next generation of observatories, including the Giant Magellan Telescope (GMT), will be sensitive enough to pick up more atmospheric traces, letting researchers characterize the composition of many more exoplanets. In particular, telescopes will look for oxygen and other molecules that exist in Earth’s atmosphere because of life. These chemicals, called “biosignatures”, don’t prove life exists on other planets, but they would be strong pieces of evidence in favor of alien life.


Baby Worlds

The planets in our Solar System are about 4.5 billion years old, which means we only have traces of evidence for what they were like when they first formed. To understand how our Solar System formed, astronomers look for newborn star systems, including baby planets.

Researchers locate these infants by finding gaps in the protoplanetary disks of matter around stars where the planets are born. The masses, orbits, and chemical compositions of new planets reveal how they develop and ultimately produce the exoplanet systems we observe around the galaxy.