How Earth-like Are Exoplanets?

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The Webb telescope will look for answers within the disks of dust and gas that surround many young stars in our galaxy.

Exploring Exoplanets Infographic
The Webb telescope is able to reveal telling details about exoplanets, which are often quite different from the planets in our own solar system. CREDIT: NASA and STScI. GET THE FULL IMAGE IN RESOURCE GALLERY >

When the Hubble Space Telescope was launched in 1990, the only planets we knew of in the entire universe were the ones in our own solar system. Now we know of thousands of “exoplanets” orbiting other stars, yet we still have much to learn about these alien worlds.

Some of the first exoplanets discovered were surprising to astronomers. Some orbit a pulsar—the remnant core of a collapsed star that creates an inhospitable environment by expelling lethal amounts of radiation into space. Many are “hot-Jupiters,” planets with the unexpected combination of being both large and yet extremely close to their host star (where most material is too hot to condense into planets). There are also “super-Earths” that are bigger than our home planet but smaller than the next-largest member of our solar system, Neptune. Exoplanets planets have also been found that are much larger than Jupiter, the largest planet in our solar system.

Astronomers have even found planets that orbit pairs of stars rather than a single star like our sun, and other planets orbiting “failed” stars known as brown dwarfs, which aren’t massive enough to produce light and energy like normal stars do.

How do such peculiar planets form? Is our own solar system, with its orderly suite of planets orbiting around an average star, unusual in the universe?

Infrared Clues

The James Webb Space Telescope will look for answers within the disks of dust and gas that surround many young stars in our galaxy. These disks are where planets are born and an infrared telescope like Webb is needed to see through the dust that blocks visible light. Webb’s infrared instruments are capable of detecting objects in these disks, observing how minuscule bits of ice and rock grow into larger bodies, and perhaps revealing how and where planets take shape.

Even fully formed planets tend to glow brighter in infrared light than in visible light, which means astronomers can use Webb to discover exoplanets that went unnoticed by visible-light telescopes. It could even provide images of young, Jupiter-like gas planets orbiting other stars.

Not only will Webb find exoplanets, but the telescope can help to reveal what these worlds are like. When a planet passes in front of its parent star, Webb can determine the planet’s size by measuring how much starlight the planet blocks. By examining starlight that passes through a planet’s atmosphere, Webb can learn what the atmosphere is made of via spectroscopy—recording the unique chemical signatures imprinted on the starlight by elements and molecules in the atmosphere. Brightness variations that Webb detects in a planet’s atmosphere could give us details about the kind of weather the planet experiences.

Hubble has helped pioneer atmospheric explorations for large, Jupiter-like planets in inhospitable environments where life—at least life as we know it—is unlikely to exist. Webb is able to explore a greater variety of planetary atmospheres, perhaps the atmospheres of super-Earths or even Earth-sized planets. Webb’s search for the chemicals of life (water, methane, oxygen, carbon dioxide, and complex organic molecules) on planets that could be friendly for alien organisms will lead us toward an answer to one of humanity’s most fundamental and profound questions: Are we alone?

Planet-forming Disk Illustration
Planets form around young stars in dusty disks, making them difficult to see in visible light. Webb’s infrared vision is able to probe the details of these disks and help us understand how planets form. CREDIT: NASA, JPL-Caltech.

Last Updated: May 31, 2018

Keywords:  Exoplanets

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