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By David McAlary
U.S. astronomers have made a finding they say increases the chance that distant stars may harbor planetary systems like our own. They have discovered two planets the size of Neptune elsewhere in our galaxy. These are the smallest planets yet found outside our solar system and may herald the discovery of even smaller ones like Earth.
Two teams of astronomers working separately found the two planets the same way most of the other 130 have been found in the past nine years. They did not see them directly, but measured the magnitude of the wobble the planets' gravity induces in their nearby stars.
The importance is that these two caused a smaller wobble in their stars than all the rest, leading the astronomers to conclude the planets are smaller. Previously detected planets have masses roughly the same as our solar system giants Jupiter and Saturn. The two new ones are estimated at roughly 20 times less mass, about that of Neptune, and only two to three times the diameter of Earth.
The findings are further evidence that planets and diversified solar systems are not rare in our galaxy and the universe beyond. One of the new planets circles a kind of star that is the most abundant to our galaxy, a so-called M class star. The planet's co-discoverer is Geoffrey Marcy of the University of California at Berkeley.
"As a result of this planet and one other around these M class stars, we estimate that something like 20 billion planetary systems exist within our Milky Way galaxy alone," he said.
The other new planet is in a system with three companion planets the mass of Jupiter, the first known multi-planet combination outside our solar system.
"It is quite a remarkable discovery," he noted. "Obviously, we're edging closer and closer to planetary systems that have a full complement like the nine planets that go around our Sun."
The researchers say they do not know the composition of the two new Neptunes, but if they are like our Neptune, it is a combination a rock and ice. Mr. Marcy's colleague, Paul Butler of the Carnegie Institution in Washington, DC, says this is a transition stage between giant gas planets the mass of Jupiter and Saturn and smaller, rocky planets like Earth and Mars, an intermediate stage that is probably found elsewhere.
"So this puts us a step closer. It puts us in that transition region where we can't quite see the Earth-like planets yet, but we are seeing their big brothers, and hopefully we will be bearing down on these smaller mass planets soon," said Mr. Butler.
The scientists say they expect that improvements in technology and search methods will lead them to Earth-size planets well within a decade.
The U.S. space agency's chief of astronomy, Anne Kinney, says the planet findings that have occurred so far reflect astounding progress since the first was detected outside our solar system in 1995.
"Ten years ago, this field barely existed. I myself never dreamed that we would be making announcements about Neptune class planets at this stage," said Ms. Kinney.
David McAlary, VOA News, Washington.
注释：
planetary 行星的
Neptune 海王星
galaxy 星系
magnitude 大小
wobble 摇晃，不稳定
gravity 重力
Jupiter 木星
diameter 直径
diversified 多变化的
Berkeley 伯克利
Carnegie Institution in Washington DC 卡耐基华盛顿研究所
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