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The search for planets. Learn how distant worlds are being discovered: Discovering New Worlds

This section has information on the transit of Venus and the historic journey of the Endeavour to observe the transit in 1769, as well as information on our search for new worlds.

Finding New Worlds

Modified from PlanetQuest

When we identified our sun as a star hundreds of years ago, humans began to wonder whether other planets might be orbiting other stars. Today we call these alien worlds orbiting stars extrasolar planets or "exoplanets."

Stars are extremely far away -- so incredibly far that any planets orbiting the stars would be too small to see and washed out by the light of the star. Because these exoplanets can't be observed directly, astronomers have sought to discern their existence by detecting their effects on the host star.

The first true exoplanet discovery came in 1994, when Alexander Wolszczan, a radio astronomer at Pennsylvania State University, discovered two or three planet-sized objects orbiting a pulsar, rather than a normal star. A pulsar is a dense, rapidly spinning remnant of a supernova explosion. Wolszczan made his discovery by observing regular variations in the pulsar's rapidly pulsed radio signal, indicating the planets' complex gravitational effects on the dead star. The origin of Wolszczan's unexpected pulsar planets remains a matter of debate.

The first discovery of a planet orbiting a star similar to the sun came in 1995. Michel Mayor and Didier Queloz found a rapidly orbiting world located blisteringly close to the star 51 Pegasi. Their planet was at least half the mass of Jupiter and no more than twice its mass. These announcements marked the beginning of a flood of discoveries. Three months later, a team led by Geoffrey Marcy and Paul Butler turned up two more planets.

By the end of the 20th century, several dozen worlds had been discovered, many the result of months or years of observation of nearby stars. Many of these newly discovered exoplanets were bizarre, with short periods and eccentric orbits close to the star. But more recently, astronomers have found planets that more closely resemble those in our outer solar system, with circular orbits and longer orbital periods.

The first planets to be found around nearby stars have never been seen. Instead, astronomers have discovered them indirectly, inferring the existence of an unseen companion through its effects on the star itself. The following is an overview of some of the planet detection methods that have thus far proved successful, as well as other methods currently in development.

Doppler Shift
This method has been the most effective so far, and has discovered many hundreds of planets. The exoplanet exerts a gravitational pull on the star that it is orbiting, causing the star to shift its position slightly; the more massive and closer the planet, the more the star will shift. Precise measurement of the velocity or change of position of a star tells us the extent of the star's movement induced by a planet's gravitational tug. Astronomers detect and measure the star's motion towards and away from Earth by analyzing the spectrum of starlight. In the Doppler shift effect, light waves from a star moving toward us are shifted toward the blue end of the spectrum. If the star is moving away, the light waves shift toward the red end of the spectrum. The larger the planet and the closer it is to the host star, the faster the star moves about the center of mass, causing a larger color shift in the spectrum of starlight. From that information, scientists can deduce the planet's mass and orbit. Many of the first planets discovered were hundreds of times more massive than Earth, with orbits very close to their parent stars -- these created the greatest Doppler shift.

Astrometric Measurement
As with the Doppler shift technique, this method depends on the slight motion of the star caused by the orbiting planet. In this method, astronomers search for the tiny observable motions of the stars on the sky. So far, no planets have been discovered by this method, but future missions may be more successful.

Transit Method
This is the method being used by the Kepler mission and by many observatories; as of December 2011, 188 exoplanets were detected using this method. If a planet passes directly between a star and an observer's line of sight, the planet blocks out a tiny portion of the star's light, reducing the star's apparent brightness. Sensitive instruments can detect this periodic dip in brightness. From the period and depth of the transits, the orbit and size of the planetary companions can be calculated. Smaller planets produce a smaller effect.

Gravitational Microlensing
Light rays become bent when passing through space that is warped by the presence of a massive object such as a star. When a planet happens to pass in front of a star along our line of sight, the planet's gravity will behave like a lens, focusing the star's light rays and causing a temporary sharp increase in brightness and change of the apparent position of the star. Networks of robotic telescopes are using this method to find exoplanets and have detected over a dozen so far.

Direct Detection
Since planets do not give off their own light, observing them directly presents formidable challenges. Arrays of telescopes working together have had some success with this method, and a new instrument has enabled telescopes to directly image exoplanets. Over two dozen planets have been detected using this method.

James Cook and the Transit of Venus

Modified from NASA Science News

Every 120 years or so a dark spot glides across the sun. Small, inky-black, almost perfectly circular, it's no ordinary sunspot. Not everyone can see it; like a solar eclipse, observers need to be within a certain region of the Earth.

On August 12, 1768, His Majesty's Bark Endeavour slipped out of harbor, Lt. James Cook in command, bound for Tahiti. Their mission was to reach Tahiti before June 1769, establish themselves among the islanders, and construct an astronomical observatory. Cook and his crew would observe Venus gliding across the face of the sun, and by doing so measure the size of the solar system. Or so hoped England's Royal Academy, which sponsored the trip.

The size of the solar system was one of the chief puzzles of 18th century science, much as the nature of dark matter and dark energy are today. In Cook's time astronomers knew that six planets orbited the sun (Uranus, Neptune and Pluto hadn't been discovered yet), and they knew the relative spacing of those planets. Jupiter, for instance, is five times farther from the sun than Earth. But how far is that ... in miles? The absolute distances were unknown.

Venus was the key. As seen from Earth, Venus occasionally crosses the face of the sun. It looks like a jet-black disk slowly gliding among the sun's true sunspots. By noting the start -- and stop-times of the transit from widely spaced locations on Earth, astronomers could calculate the distance to Venus using the principles of parallax. The scale of the rest of the solar system would follow.

But there was a problem. Transits of Venus are rare. They come in pairs, eight years apart, separated by approximately 120 years. An international team did try to time a Venus transit in 1761, but weather and other factors spoiled most of their data. If Cook and others failed in 1769, every astronomer on Earth would be dead before the next opportunity in 1874.

The crew arrived mostly intact at Tahiti on April 13, 1769, almost two months before the transit. On June 3, 1769, Venus' little black disk could be seen gliding across the blinding sun through special telescopes brought from England.

Cook and ship astronomer Charles Green observed the "black drop effect." When Venus is near the limb of the sun -- the critical moment for transit timing -- the black of space beyond the sun's limb seems to reach in and touch the planet. The black drop effect and the fuzziness of Venus' atmosphere made it hard to say just when the transit began or ended. This was a problem for observers elsewhere, too, not only Cook in Tahiti. Observations of Venus' 1769 transit from 76 points around the globe, including Cook's, were not precise enough to set the scale of the solar system. Astronomers didn't manage that until the 19th century when they used photography to record the next pair of transits.

On July 11, 1771, Cook returned to England at Deal. The surviving crew of the Endeavor had circumnavigated the globe, catalogued thousands of species of plants, insects and animals, encountered new (to them) races of people, and hunted for giant continents. It was an epic adventure.

More details about the history of the transit of Venus is available at the Sun-Earth Day website.

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Exoplanets: A play in four parts

Part I: How do we find Exoplanets? What have we found?
Part II: How do we know what individual exoplanets are like?
Part III: What have we learned about individual exoplanets so far?
Part IV: What are the future plans for studying exoplanets?
Questions and Answers

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Discovering New Worlds Videos

Titles and Introduction
Solar Eclipses
Lunar Eclipses
Transit of Venus and Mercury
Occultations Reveal Planets Around other Stars
Occultations and Transits Provide New Science
Questions and Answers
Education and Public Outreach

All Topics
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Last Updated: 12 Sep 2014