VOA慢速英语--科学家发现太阳系中最古老的天体（在线收听）

 ‘Alien Asteroid’ Could Be Oldest Object in Our Solar System

Astronomers have been closely watching an object that travels in an orbit around our sun.

Astronomers say this object -- an asteroid -- circles the sun in the same orbital path as the planet Jupiter, but moves in the opposite direction.

They say the asteroid came from somewhere else in the universe. And now some experts believe it could be be the oldest object in our solar system.

In late May, researchers reported on a close examination of the asteroid and its orbit. They said that evidence suggests the asteroid formed elsewhere and was captured by gravitational forces when our solar system was formed.

Their report was published in the Monthly Notices of the Royal Astronomical Society: Letters.

The researchers believe the sun, planets and other objects in our solar system formed from a swirling cloud of gas and dust. They say this took place about 4.5 billion years ago.

Fathi Namouni is an astronomer with the Observatoire de la C?te d’Azur in France. He was the lead writer of the report on the asteroid.

Namouni told the Reuters news service that the asteroid is “a strong candidate for the oldest object in the solar system.”

The asteroid has a long, somewhat complex name. It is called (514107) 2015 BZ509, or “BZ” for short. It is nearly 3 kilometers wide. Scientists say they are unsure about the materials it is made of.

BZ travels in the opposite direction of all the planets and nearly everything else in our solar system. This kind of movement is called a retrograde orbit.

"How the asteroid came to move in this way, while sharing Jupiter's orbit, has until now been a mystery," Namouni explained. "If 2015 BZ509 were a native of our system, it should have had the same original direction as all of the other planets and asteroids."

BZ is not the first object that scientists recognize as having travelled from a different star system into our own. The first-known alien traveler was a large object called ‘Oumuamua.’ Researchers observed it passing through our solar system last year.

But Namouni said BZ and Oumuamua are very different. Oumuamua simply passed through our solar system, while BZ has been orbiting the sun for a long time.

Scientists have observed BZ with ground-based telescopes in the American states of Hawaii and Arizona. They said BZ apparently left its home star system when the star system interacted with other systems in a tight fitting star cluster.

“The solar system formed in a star cluster where each star has its own planets and asteroids. The close proximity of the star systems, assisted by their gravitational interactions, helped remove and capture asteroids,” Namouni said.

Helena Morais helped with the new research.

Morais is an astronomer at the Universidade Estadual Paulista in Brazil. She said the movement of BZ is similar to that of the planet Jupiter.

“They complete an orbit around the sun in the same amount of time while moving in opposite directions,” she said.

BZ may be important in discussions about the development of life on Earth.

“This discovery tells us that the solar system is likely to be home to more extra-solar asteroids and comets captured early in its history. Some of these objects may have collided with the Earth in the past possibly carrying water, biomolecules or even organic material,” Morais added.

The researchers said a number of other solar system objects with retrograde orbits are being investigated as possible “alien asteroids.”

I’m Pete Musto.

Words in This Story

asteroid – n. any one of thousands of small planets that circle around the sun

solar system – n. a star other than our sun and the planets that move around it

swirl(ing) – v. to move in circles or to cause something to move in circles

alien – adj. from somewhere other than the planet Earth

interact(ed) – v. to come together and have an effect on each other

fit(ting) – v. to be the right size and shape for someone or something

cluster – n. a group of things or people that are close together

proximity – n. the state of being near

comet(s) – n. an object in outer space that develops a long, bright tail when it passes near the sun

collide(d) – v. to hit something or each other with strong force