What is an exoplanet? – Exploration of exoplanets: planets beyond our solar system


What is an exoplanet?

Fast facts

What is an exoplanet?

An exoplanet is any planet located beyond our solar system. Most orbit other stars, but floating exoplanets, called rogue planets, orbit the galactic center and are not attached to any stars.



Most The exoplanets discovered so far are found in a relatively small region of our galaxy, the Milky Way. We know from NASA’s Kepler Space Telescope that there are more planets than stars in the galaxy.

By measuring the sizes (diameters) and masses (weights) of exoplanets, we can see compositions ranging from very rocky (like Earth and Venus) to very gas-rich (like Jupiter and Saturn). Exoplanets are made up of similar elements to planets in our solar system, but their mixtures of these elements may differ. Some planets can be dominated by water or ice, while others are dominated by iron or carbon. We have identified lava worlds covered with molten seas, planets swollen with the density of styrofoam, and dense cores of planets orbiting their stars.

There is a wide variety among exoplanets – planets outside of our solar system. There are aquatic worlds, lava planets, egg-shaped worlds, planets with multiple suns, and even planets with no sun at all! What can we learn from all of this weird and wonderful variety? What does it tell us both about the exoplanets themselves and about our own home planet? Credit: NASA / JPL-Caltech

The first exoplanets were discovered in the 1990s and since then we have identified thousands of them using various detection methods. It is quite rare for astronomers to see an exoplanet through their telescopes the way you might see Saturn through a telescope from Earth. This is called direct imagery, and only a handful of exoplanets have been found this way (and these are usually young gas giant planets orbiting very far from their stars).

We now live in a universe of exoplanets. The number of confirmed planets is in the thousands and increasing. This is only from a small sample of the galaxy as a whole. The number could reach tens of thousands within a decade, as we increase the number and observational power of robotic telescopes projected into space.

The Nancy Grace Roman Space Telescope, formerly known as WFIRST, is a future space telescope designed to perform wide-field imaging and spectroscopy of the infrared sky. One of the objectives of the Roman Space Telescope will be to search for clues about dark energy, the mysterious force that accelerates the expansion of the universe. Credit: NASA’s Goddard Space Flight Center

Most exoplanets are discovered by indirect methods: measuring the gradation of a passing star, called the transit method, or monitoring a star’s spectrum for telltale signs of a planet pulling on its star and causing a slight Doppler shift of its lumen. Space telescopes have found thousands of planets by observing “transits,” the slight decrease in light from a star as its tiny planet passes between it and our telescopes. Other detection methods include the gravitational lens, known as the “oscillation method”.

Credits: NASA / W. Stenzel

But when several methods are used together, we can learn the vital statistics of entire planetary systems – without ever directly imagining the planets themselves. The best example to date is the TRAPPIST-1 system about 40 light years away, where seven planets roughly the size of Earth orbit a small red star.

The TRAPPIST-1 planets have been examined with terrestrial and space telescopes. Space studies have revealed not only their diameters, but the subtle gravitational influence that these seven tightly packed planets have on each other; from there, scientists determined the mass of each planet.

We now know their masses and diameters. We also know how much energy radiated by their star hits the surfaces of these planets, allowing scientists to estimate their temperatures. We can even make reasonable estimates of the light level, and guess the color of the sky, if you were standing on one of them. And although much remains unknown about these seven worlds, including whether they have atmospheres or oceans, ice caps or glaciers, it has become the most well-known solar system outside of our own.

  • artist concept of different sizes and colors of planets

    Types of planets

    Exoplanets come in a wide variety of sizes, from gas giants larger than Jupiter to small, rocky planets about as large as Earth or Mars. They can be hot enough to boil metal or locked in a freezer. They can orbit their stars so closely that a “year” lasts only a few days; they can orbit two suns at the same time. Some exoplanets are sunless thieves, roaming the galaxy in permanent darkness.

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Next: Exoplanets – In depth


Arline J. Mercier

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