人类的两个研究发现之一就是对天外星系的观测和研究，而如果能够发现新的行星则对扩展人类的视野会产生巨大的推动作用。近日科学家们又发现了一颗新的行星。这究竟是怎么回事呢？

　　If three's a crowd, a new planet recently discovered orbiting a sunlike star is really cluttering up its neighborhood.

　　The new planet is the fourth Jupiter-like world to be found around the young star HR 8799, astronomers announced today. The same team had previously found the other three planets in 2008, when they took a direct picture of the star system.

　　Of the more than 500 planets discovered to date outside our solar system, most have been found via indirect methods, such as looking for planets' gravitational tugs on their host stars or for dips in starlight when planets pass in front of their hosts.

　　The new planet was also found in a direct image of the HR 8799 system. But based on the masses of the planets and their distances from the star, the fourth world challenges current theories of planet formation, according to the study authors.

　　"This is the first multiplanet system directly imaged so far, so it's quite a feat," said lead study author Christian Marois, an astronomer at the Herzberg Institute of Astrophysics in Canada.

　　"But we are now stuck with four planets [and] we cannot explain their formation and their current locations by any of our models."

　　Fourth Planet Too Close to Its Parent?

　　The previously known planets around HR 8799 are about five, seven, and ten times Jupiter's mass. They orbit between 2.2 billion miles (3.5 billion kilometers) from the star—roughly the same as Neptune's distance from the sun—and 6.3 billion miles (10.1 billion kilometers), or almost twice Pluto's distance.

　　The most distant planet circles just inside a dusty disk similar to that produced by the solar system's Kuiper belt, a region of small, icy bodies that includes Pluto.

　　The whole HR 8799 system is estimated to be no more than 60 million years old, so the planets are still glowing with heat from their formation. This heat signature is what betrayed the planets' positions in the previous infrared images of the star.

　　Astronomers found the fourth planet using the Keck II telescope on the peak of Mauna Kea in Hawaii. The instrument is equipped with a special near-infrared imager and adaptive optics to eliminate the blurring effect of Earth's atmosphere.

　　Images from Keck II show that the newfound planet is also a gas giant, about seven times Jupiter's mass. But it orbits closer to the star, at a mere 1.4 billion miles (2.2 billion kilometers), equivalent to between the orbits of Saturn and Uranus.

　　The new Keck data also indicate there may be an asteroid belt just inside the newfound planet's orbit.

　　Widespread "Jupiters" Creating Dilemma

　　Overall, the HR 8799 system is starting to look like a scaled-up version of our solar system, the astronomers note. But the setup doesn't fit either of the two currently accepted models for how gas giant planets form, Marois said.

　　It's thought planets in general form out of the disks of gas and debris that surround many young stars. One model for how gas giants form is called gravitational instability, in which a perturbation in the disk causes a clump of material to suddenly coalesce into a gassy planet.

　　The other model is core accretion, in which material first clumps into a rocky core, and the core then gravitationally gathers nearby gases. (See "New Model of Jupiter's Core Ignites Planet Birth Debate.")

　　According to Marois's team, the problem with such widespread gas giants is that they all couldn't have formed the same way. Too far from the star and there's not enough gas for core accretion to work, but too close and it's too hot for debris to become gravitationally unstable.

　　"The system is either just too young and the planets are just too far away and don't have time to form before the gas in the disk is depleted, or they are too close and the disk is too warm to form planets," Marois said.

　　Gas giant planets have been found in orbits close to their parent stars, sometimes even closer than Mercury is to the sun. In these cases, many experts believe the so-called hot Jupiters may have formed farther away from their stars and then migrated closer over time.

　　"That is one possible way out in explaining the problem. There is probably more to it than that—but this will require more work," Marois said.

　　"But the fact that we are seeing this planetary system at this early stage of evolution is definitely giving us insight on what happened to arrive at its current configuration."

　　The HR 8799 planetary system is described in a paper published online this week by the journal Nature.

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