An Introduction to Exoplanets

Distant worlds called "exoplanets" can be detected as they pass between us and their star. The time they take to do this tells us about their orbits. Starlight filtering through the atmosphere can reveal what gases are there. This is a Neptune-size planet orbiting its star, as depicted by a space artist using actual NASA data. NASA

Did you ever look up at the sky and think about worlds circling the distant stars? The idea has long been a staple of science fiction stories, but in recent decades, astronomers have discovered many, many planets "out there". They're called "exoplanets", and by some estimates, there could be close to 50 billion planets in the Milky Way galaxy. That's just around the stars that might have conditions that could support life. If you add in all the types of stars that may or may not have habitable zones, the count is much, much higher. However, those are estimates based on the actual number of known and confirmed exoplanets, which is more than 3,600 worlds around stars that have been observed by several efforts, including the Kepler Space Telescope exoplanet search mission and a number of ground-based observatories. Planets have been found in single-star systems as well as in binary star groupings and even in star clusters.

The first exoplanet detection was made in 1988, but not confirmed for a few years. After that, detections began to occur as telescopes and instruments improved, and the first planet known to orbit a main-sequence star was made in 1995. The Kepler Mission is the grande dame of exoplanet searches, and has observed thousands of planet candidates in the years since its 2009 launch and deployment. The GAIA mission, launched by the European Space Agency to measure positions and proper motions for stars in the galaxy, is providing useful maps for future exoplanet searches.

What are Exoplanets?

The definition of exoplanet is pretty simple: it's a world orbiting another star and not the Sun. "Exo" is a prefix that means "from outside", and perfectly describes in one word a pretty complex set of objects that we think of as planets. There are many types of exoplanets — from worlds similar to Earth in size and/or composition to worlds more like the gas giant planets in our own solar system. The smallest exoplanet is just a couple of times the mass of Earth's moon and orbits a pulsar (a star that gives off radio emissions that pulsate as the star turns on its axis). Most planets are in the "middle" of the size and mass range, but there are some pretty big ones out there, too. The most massive one found (so far) is called DENIS-P J082303.1-491201 b, and it appears to be at least 29 times the mass of Jupiter. For reference, Jupiter is 317 times the mass of Earth. 

What Can We Learn about Exoplanets?

The details that astronomers want to know about distant worlds are the same as for the planets in our own solar system. For example, how far away do they orbit from their star? If a planet lies at the right distance that allows liquid water to flow on a solid surface (the so-called "habitable" or "Goldilocks" zone), then it's a good candidate to study for signs of possible life elsewhere in our galaxy. Just being in the zone doesn't guarantee life, but it gives a world better chances to host it. 

Astronomers also want to know if a world has an atmosphere. That's important for life as well. However, since the worlds are quite far away, atmospheres are nearly impossible to detect just by looking at the planet. One very cool technique allows astronomers to study light from the star as it passes through the planet's atmosphere. Some of the light is absorbed by the atmosphere, which IS detectable using specialized instruments. That method shows which gases are in the atmosphere. The temperature of a planet can be measured, and some scientists are working on ways to measure a planet's magnetic field as well as the chances that (if it's rocky) it has tectonic activity. 

The time it takes for an exoplanet to go around its star (its orbital period) is related to its distance from the star. The closer it orbits, the faster it goes. A more distant orbit moves more slowly. Many planets have been found that orbit quite quickly around their stars, which raises questions about their habitability since they might be warmed too much. Some of those fast-moving worlds are gas giants (rather than rocky worlds, as with our own solar system). That led scientists to speculate about where planets form in a system early in the birth process. Do they form close to the star and then migrate out? If so, what factors influence that motion? This is a question we can apply to our own solar system, as well, making the study of exoplanets a useful way to look at our own place in space, too.

Finding Exoplanets

Exoplanets come many flavors: small, large, giants, earth-type, superJupiter, hot Uranus, hot Jupiter, super-Neptunes, and so on. The larger ones are easier to spot on initial surveys, as are the planets that orbit far from their stars. The real tricky part comes when scientists want to search for close-in rocky worlds. They are quite challenging to find and observe.

Astronomers long suspected that other stars could have planets, but they faced major hurdles in actually observing them. First, stars are very bright and large, while their planets are small and (in comparison to the star) rather dim. The star's light simply hides the planet, unless it's pretty far away from the star (say about the distance of Jupiter or Saturn in our solar system). Second, stars are distant, and that also makes small planets very difficult to spot. Third, it was once assumed that not all stars would necessarily have planets, so astronomers focused their attention on stars more like the Sun. Today, astronomers rely on the data coming from Kepler and other large-scale planet searches to identify candidates. Then, the hard work begins. Many follow-up observations have to be made to confirm the existence of a planet before it's confirmed.

Ground-based observations teased out the first exoplanets starting in 1988, but the true search began when Kepler Space Telescope was launched in 2009. It looks for planets by watching the brightness of stars over time. A planet orbiting the star in our line of sight will cause the brightness of the star to dim a tiny bit. Kepler's photometer (a very sensitive light meter) detects that dimming and measure how long it takes as the planet "transits" across the face of the star. The process for detection is called the "transit method" for that reason.

Planets can also be found something called "radial velocity". A star can be "tugged on" by the gravitational pull of its planet (or planets). The "tug" shows up as a slight "shift" in the star's spectrum of light and is detected using a special instrument called a "spectrograph". This is a good discovery tool, and is also used to follow up on a detection for further investigation.

The Hubble Space Telescope has actually photographed a planet around another star (called "direct imaging"), which works well since the telescope can zero its view into the small area around a star. This is nearly impossible to do from the ground, and is one of a handful of tools to help astronomers confirm the existence of a planet.

Today there are nearly 50 ground-based exoplanet searches going on, plus two space-based missions: Kepler and GAIA (which is creating a 3D map of the galaxy). Five more space-based missions will fly in the next decade, all expanding the search for worlds around other stars.

Ultimately, the search for planets can and will lead to the search for life on those worlds. It may not be life as we know it, and probably won't be "aliens" as some imaginative folks who are UFO fanatics would like them to be, but it will be live. Finding it will make a profound difference in our understanding of the universe.