20 Years of Exoplanets: The Search for Worlds Beyond Our Own

Special Feature

By NASA’s Amazing Space reporters October 2015

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Credit: ESO/M. Kornmesser/Nick Risinger (skysurvey.org)

A P U B L I C A T I O N O F N A S A ’ S “A M A Z I N G S P A C E ” E D U C A T I O N P R O G R A M

WITNESSSTARTHE

National Aeronautics and Space Administration

As fAr bAck As Ancient Greece, humans have

wondered whether other

solar systems might exist in the

universe. in about 300 bce, Greek

philosopher epicurus, for example,

proposed that the universe’s

boundaries were limitless and that

space was full of other worlds.

Another Greek astronomer of that

era, Aristarchus of samos, had

proposed that the sun was the center

of the solar system, and that the stars

might be other suns.

However, those ideas faded away,

and the earth-centered solar system

became the common belief in

astronomy for thousands of years.

the sun and the stars, it was thought,

orbited our planet. in 1543, Polish

astronomer nicolaus copernicus

developed the mathematics of the

Extrasolar planet at 51 Pegasi: This artist’s view shows a giant extrasolar planet orbiting the star 51 Pegasi, located 51 light-years from Earth. Astronomers using ground-based telescopes found the planet in 1995, marking the first extrasolar planet discovered around a star similar to our sun. The planet is roughly half the size of Jupiter and is orbiting dangerously close to its star.

sun-centered solar system and caused

a great shift in scientists’ view of

the universe. His work led to the

recognition that earth does indeed

orbit the sun and to the follow-on

speculation that other stars had

planets circling them.

the invention of the telescope in the

early 1600s put our solar system into

even clearer focus, revolutionizing

astronomy. Galileo was one of the

first to use the telescope to search

the heavens. Among his findings

was spotting the four largest moons

orbiting around Jupiter. Over the next

two centuries, bigger and sharper

telescopes allowed astronomers to

make more solar-system discoveries,

including finding the planets Uranus

and neptune orbiting the sun.

the early 1900s saw another large

leap in our understanding of earth’s

place in the universe. Using the

Hooker telescope — the largest

telescope of its day — at Mt. Wilson

Observatory in california, astronomer

edwin Hubble found that some

nebulae, such as the Andromeda

nebula, were separate galaxies like

our own Milky Way galaxy. because

each galaxy contains hundreds of

billions of stars, the potential for

other worlds orbiting other stars

vastly increased.

The hunt pays off

Until the 1990s, however,

astronomers had not found

any planets circling other suns.

Uncovering planets outside our

solar system, or extrasolar planets, is

extremely difficult because they are

too small, dim, and far away to be

directly imaged by telescopes. to find

them, astronomers had to develop

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other methods, such as detecting

their effects on their host star.

in 1995, astronomers using ground-

based telescopes finally nabbed an

extrasolar planet that was orbiting

a star similar to our sun. the planet

was roughly half the size of Jupiter

and was orbiting dangerously close

to its star, called 51 Pegasi. the

system is located about 51 light-years

from earth. (nAsA’s Jet Propulsion

Laboratory is commemorating the

20-year anniversary of the first

extrasolar planet discovery around

a sun-like star with several special

events.)

the planet detection around 51

Pegasi began a gush of extrasolar

planet discoveries. Astronomers

attributed the boost in discoveries

partly to improvements in

technology. significant improvements,

for example, were made in

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spectrometers, instruments that

separate starlight into its component

colors for analysis. spectrometers can

help astronomers detect extrasolar

planets and sample the chemical

elements in the planet’s atmosphere.

faster computers and more

sophisticated computer software also

helped astronomers readily identify

slight changes in light from stars to

find planets circling them.

Space missions boost the extrasolar planet count

At the start of the twenty-first

century, several dozen planets

outside our solar system had been

found using ground-based telescopes.

in 2006, scientists launched the

french corot mission, the first space

mission dedicated to the extrasolar

planet hunt. the mission helped

astronomers discover dozens more

extrasolar planets.

Earth Earth

Moon Moon

Mercury Mercury

VenusVenus SunSun

MarsMars

Jupiter Jupiter

EARTH-CENTERED SOLAR SYSTEM SUN-CENTERED SOLAR SYSTEM

Saturn Saturn

Earth Earth

Moon Moon

Mercury Mercury

VenusVenus

SunSun

MarsMars

Jupiter Jupiter

Saturn Saturn

Historical models of the solar system

Space Telescope Science Institute, Graphics Dept.

These simplified diagrams show an Earth-centered and a sun-centered solar system. In the ancient world, people thought that the sun and other objects orbited Earth (shown in the diagram at left). However, Polish astronomer Nicolaus Copernicus’ models of a sun-centered solar system (shown in the diagram at right) ignited a change in that thinking in 1543.

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Andromeda

Another step forward in the search

for extrasolar planets began with the

launch in 2009 of the kepler space

observatory, nAsA’s first extrasolar

mission. kepler’s goal was to search

hundreds of thousands of stars for

signs of planets.

the spacecraft has found a diverse

array of planets, including exotic,

multi-planet solar systems. More

importantly, kepler has found that

smaller planets (earth-sized rather

than Jupiter-sized) are likely to be

the most common in the Milky Way

galaxy. kepler uncovered several

planets that are about the same size

as earth and orbit within the star’s

habitable zone, where liquid water,

which is necessary for life, can exist.

Overall, kepler has added more

than 1,000 confirmed planets to the

extrasolar planet count, which now

stands at nearly 2,000.

Astronomers also have used nAsA’s

spitzer space telescope, which

observes in infrared light, to study

extrasolar planets. in 2007, the

spitzer telescope detected water

vapor for the first time on another

world.

Hubble contributes to the search

nAsA’s Hubble space telescope

has made some unique

contributions to the planet

hunt. Hubble made one of the

first visible-light images of an

extrasolar planet circling the

nearby, bright star fomalhaut,

located 25 light-years from earth.

the planet is orbiting about 10

times farther away from its star

than saturn is from the sun.

Hubble also helped astronomers

analyze the atmospheres of

extrasolar planets. Hubble made

the first measurements of the

makeup of the atmospheres

of two Jupiter-sized extrasolar

planets. the giant planets are

orbiting too close to their stars

and are therefore too hot for life.

but the Hubble observations

demonstrate that the basic

chemistry for life can be measured

on planets orbiting other stars.

even if a planet is rocky, earth-

sized, and just the right distance

from its star, it also must have

an atmosphere that contains the

correct ingredients to foster and

sustain life. Venus, for example, is

earth’s twin by size, but its dense

atmosphere is made up mostly of

carbon dioxide, which helps keep

the planet sizzling hot. Venus,

therefore, is not hospitable for life.

the distant worlds observed by

Hubble and other telescopes

are just the tip of the iceberg,

according to a Hubble survey of

the center of our Milky Way.

the study revealed that our

galaxy should be brimming with

Our Milky Way galaxy’s nearest large neighbor is the Andromeda galaxy, shown in this image. In the early 1900s, scientists generally thought that Andromeda was a nebula within our Milky Way. Astronomer Edwin Hubble’s observations in the 1920s revealed that Andromeda and other similar nebulae are separate galaxies like our own Milky Way galaxy.

Credit: Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF

see MOre Hubble images and read more

star Witness news stories at Amazing space,

nAsA’s award-winning educational website

for k-12 students and teachers. amazing-space.stsci.edu

www.nasa.gov

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100 billion planets, at least one planet

for every star.

The future looks bright

Within our own solar system,

astronomers are still learning new

information. nAsA’s new Horizons

mission, for example, was the first

spacecraft to visit Pluto in the outer

solar system. the spacecraft’s images

of Pluto show a surprisingly complex

world with giant ice mountains that

had never been seen before.

the study of worlds outside our solar

system will be continued by future

spacecraft, such as nAsA’s transiting

exoplanet survey satellite (tess)

and James Webb space telescope.

tess will find planets in the same

manner as kepler, but search stars

that are closer to our sun. the Webb

telescope will use infrared light to

examine in detail the extrasolar

planetary systems found by tess

and other telescopes. it will even

study the chemical makeup of an

extrasolar planet’s atmosphere. Who

knows what new discoveries these

observatories will uncover?

Looking to the far future, it is clear

that planets in the universe are really

just getting started. A new analysis

of data from Hubble and kepler

considers the expected rate of new

planet formation over the entire

predicted lifetime of our galaxy. Our

planet earth formed relatively early

in that context. About 90 percent of

the planets that ever will form are yet

to be born. With roughly 100 billion

planets in our galaxy, 100 billion

galaxies in the universe, and trillions

of years ahead, planet studies in the

universe have vast possibilities across

both space and time.

Credit: NASA, ESA, and G. Bacon (STScI)

Extrasolar planet HD 189733b: This artist’s illustration depicts extrasolar planet HD 189733b, with its parent star peeking above its top edge. Astronomers used the Hubble Space Telescope to detect methane and water vapor in the Jupiter-sized planet’s atmosphere. They made the finding by studying how light from the star filters through the planet’s atmosphere.