Moreover, as astronomers keep reminding us, humanity will eventually lose Earth as its home, whether because of global warming or the ultimate plague or a killer asteroid or the Sun’s inevitable demise. Before then, if we want the universe to remember us or even know we were here, we need to get away.

It was only in 1995 that a team of Swiss astronomers led by Michel Mayor of the Geneva Observatory discovered the first planet of another Sun-like star using what is now known as the “wobble” method. A planet gives its star a little gravitational tug as it goes around, causing the star to go back and forth, or wobble, a little as both star and planet circle the same center of gravity. They detected a wobble in the motion of the star 51 Pegasi as an object about half the mass of Jupiter whipped around it every four days.

Like Olives in a Martini Glass

Over the next decade, Dr. Mayor’s group and another planet-hunting team led by Dr. Marcy and R. Paul Butler of the Carnegie Institution leapfrogged each other in finding exoplanets, as they are called. More and more astronomers have joined the hunt, discovering smaller and smaller planets. Astronomers have recorded direct images of four planets swirling like olives in a martini glass around a star known as HR 8799, 130 light-years from Earth in the constellation Pegasus, and another circling Fomalhaut, only 25 light-years from Earth, in the constellation Piscis Austrinus.

There are now more than 500 planets listed on the Jet Propulsion Laboratory’s PlanetQuest Web site. None are habitable.

Among them is the so-called Styrofoam planet  an early trophy of Kepler’s  a planet that is again half as large as Jupiter, but so puffed up by the heat of its star that it is only one-tenth as dense. Another is a planet composed almost entirely of superheated water and sometimes called the Steam World; it is known as Gliese 1214b, about 40 light-years from here in the constellation Ophiuchus.

Last year, a team of American astronomers announced that they had discovered a Goldilocks planet orbiting a dim red dwarf star at just the right distance to harbor water on its surface, making it a potential site for life. Gliese 581g, as it is known, is part of the Gliese 581 system 20 light-years from here, in Libra. But then the Swiss astronomers who first spotted that system were not able to find the Goldilocks planet in their own data, causing many astronomers, but not its discoverers, to doubt that the friendly 581g was real.