This technique is based on the idea that if an unseen planet orbits a star, its gravitational pull causes a slight "wobble" in the light wavelengths coming from that star.
In addition to revealing the 28 new planets, advances in this technique recently allowed a postdoctoral astronomer at the University of Geneva in Switzerland to pin down the size of a large exoplanet that was discovered almost two years ago by Marcy's team.
This planet, which is about 22 times the size of Earth, orbits a star called Gliese 436 that sits some 30 light-years away.
From the planet's size and mass, scientists were able to calculate its density, which turns out to be similar to Neptune's.
"This planet has a composition made of rock in its core and water compressed into a solid form in a surrounding envelope around that core … with perhaps small amounts of hydrogen and helium [on its surface]," Marcy said.
"So we're actually getting a very clear sense of the composition of a planet orbiting another star for the first time, and that it has water," he said.
"And of course water is the essential ingredient for life here on Earth."
Today more powerful technology is also enabling scientists to spot smaller gas giants than they have seen before.
"We're just now getting to the point where, if we were observing our own solar system from afar, we would be seeing Jupiter," Jason Wright, a UC Berkeley postdoctoral fellow, said at the AAS meeting.
But the Doppler technique can only be advanced so far and may never be strong enough to find an Earth "twin."
"I am skeptical that the Doppler method will ever be able to find a terrestrial planet around a sunlike star," Charles Lineweaver, a senior fellow at the Planetary Science Institute in Weston, Australia, commented via email.
"For that we need to wait for the next generation of space-based missions … ."
Finding Other "Earths"
Based on data gathered so far, of the more than 200 billion stars in the Milky Way galaxy, at least 10 percent are thought to have planetary systems, UC Berkeley's Marcy said.
And at least 30 percent of all stars that are known to host planets have more than one, scientists say.
"Many of them remind us of our home solar system," Marcy said (explore a virtual model of our solar system).
"We're finding a lot of cases in which the larger planets—the Jupiters and the Saturns—orbit further from the star than the smaller planets, and that is in fact the case for our solar system," he said.
Among this year's exoplanet finds are at least four new multiple-planet star systems.
Astronomers are finding that stars harboring the most planets are those that are rich in heavier elements, such as silicon, oxygen, iron, and nickel.
This may give researchers a clue as to where to look for possible habitable worlds.
"It's those richer stars that we're focusing our attention on, because those heavy elements are the building blocks of rocky planets like our Earth," Marcy said.
After all, he said, "it's not just the planets that require the heavy elements, it's the organisms themselves, should any exist."
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