3 bright planets star in night sky
Three bright planets await your view at dusk. They're easy to spot just by looking. Over 100 more planets populate the evening sky unseen; they have been discovered in just the last 12 years orbiting nearby stars. And these, surely, are just the nearest of countless billions of planets accompanying stars as far as the eye (or telescope) can see.
The brightest planet is Venus, shining low in the west in twilight. Look early, around 9 to 9:20 p.m. this week, before it gets too low and sets.
As twilight deepens, look to the right of Venus for Saturn, less than 1 percent as bright. Each day Saturn moves farther off to the right of Venus (they appeared closest together on June 30). Saturn looks so much fainter because it's 20 times farther away. Both sink lower toward the horizon every day.
Also in the vicinity are the stars Regulus and Algieba, a trace fainter than Saturn. They sink lower day by day at a faster rate. The scenes here show how the configuration changes during the next 10 days.
The third naked-eye planet at dusk is Jupiter, shining brightly in the south. Unlike Venus and Saturn, Jupiter remains in good view long into the night.
To the lower right of Jupiter you'll see an orange-red firespark: the star Antares. It's the leading light of the constellation Scorpius, whose other stars sparkle below Antares and to its upper right.
The other planets of our solar system either don't rise until early morning (Mars, Mercury) or are too faint to see in the evening without optical aid (Uranus, Neptune, and Pluto, if you count it).
Until recently, that was it for planets. By the 1990s, astronomers had excellent reasons to think that planets were fairly common around other sunlike stars, but no one knew for sure. The stars were much too far away for the world's best telescopes to have a hope of showing such tiny specks next to the dazzle of their suns.
Then in 1995, two teams of astronomers almost simultaneously discovered the first extrasolar planets around sunlike stars, using a new technique. The breakthrough came from instruments that could measure Doppler shifts in starlight (the shortening or lengthening of light waves due to motion) to an accuracy of one part in a hundred million. This enabled the astronomers to measure the velocity of a star toward or away from their telescopes to the incredible accuracy of just a few miles per hour, mere jogging speed.
Included in the measured velocity are the ever-changing motions of the telescope itself, caused by the Earth's rotation, the Earth's motion around the sun, the slight gravitational influences of other solar system bodies on the Earth and Sun, and the Sun's own motion through space. But these are precisely known. Subtract them, and you're left with the star's speed alone.
In some cases, if a giant planet orbits a star it can exert enough of a gravitational tug to make the star change velocity, as the planet goes around it, by at least a few miles per hour. At last, planets of other stars were in range of discovery.
It was a huge achievement. Extrasolar planet discoveries by the star-wobble method began pouring in. Today the number of exoplanets stands at 241 (including a few found by other cutting-edge methods), and new ones are announced every few weeks.
Each one is a unique world. The wobble technique tells not just that a planet is orbiting a star; it also tells the planet's mass (at least roughly), how fast it orbits, the orbit's size and shape, and hence the planet's approximate temperature. The Extrasolar Planets Encyclopedia (on the Web at exoplanet.eu) lists a huge variety of worlds. They range from 3,000 or more Earth masses down to a mere 5 to 8 Earth masses, the current lower limit of the wobble method. Some are red-hot, some are deep-frozen, and a few are shirt-sleeves-clement.
Low-mass worlds like Earth still remain beyond reach. But they shouldn't be for long. In our lifetime we'll probably be able to point to the locations of known earthlike worlds deep in the evening sky. 