Planets to planets, dust to dust

Planets in the galaxy are colliding and producing microscopic dust particles that are now combining to form other rocky planets, according UCLA astronomers.

UCLA astronomers recently discovered formations of rocky terrestrial planets around the Pleiades star cluster by observing the accumulation of dust particles from collisions.

“What is special (in the Pleiades) is the amount of dust that resulted. It’s actually very complex, indicating collision,” said Eric Becklin, a physics and astronomy professor emeritus.

This is the first clear evidence for planet formation in the Pleiades, and it indicates that other planets like those may be common.

The Pleiades star cluster is about 400 light years away ““ one of the closest star clusters to Earth. It is easily visible in the evening sky and is probably the most striking to the naked eye, said Benjamin Zuckerman, a physics and astronomy professor emeritus.

Zuckerman led the research with Inseok Song, a scientist at NASA’s Spitzer Science Center at the California Institute of Technology, and Joseph Rhee, a UCLA postdoctoral scholar in astronomy. They utilized the Gemini Observatory in Hawaii and the Spitzer Space Telescope in Pasadena.

The Spitzer and the Gemini North telescopes allowed for the identification of the amount and location of dust around the star, measuring the heat radiation from the dust which emerges at infrared wavelengths, Zuckerman said. Infrared radiation has electromagnetic radiation wavelengths longer than that of visible light, but shorter than that of microwaves.

The scientists were able to observe the countless microscopic dust particles around young stars. The dust is a remnant of the collision. Because of the presence of hot dust particles, it was concluded that collisions with larger objects had occurred. These particles may form new planets, said Kevin McKeegan, professor of geochemistry.

When higher heat is present, dust particles may be present as they emit shorter wavelengths. The shorter the wavelength, the greater the heat the object radiates. Also, hotter wavelengths generally indicate more recent collisions, Zuckerman said.

Very bright infrared sources represent the presence of a lot of dust, he said.

This process of dust particle growth gradually turning into planets is called accretion, a process in which the size of something gradually increases by steady addition of smaller parts.

In this case, accretion starts when small particles stick together and grow. Bigger objects have more violent collisions, McKeegan said.

It takes 10,000 to 100,000 years to gather the clusters of dust particles into planetesimals, or tiny planets, provided that there is enough gas so the particles can stick together.

Then, the planetesimals turn into planetary embryos after collisions. Planetary embryos are about the size of Earth’s moon, Zuckerman said.

Once the accumulation of dust particles start to get larger and larger, gravity begins to help the particles stick together more, McKeegan said.

“The whole process takes probably an order of a few million years,” he said.

The creation of the terrestrial planets are around stars that are more than 100 million years old. The very small dust particles disappear as they become part of the potential planet, McKeegan said.

Some astronomers believe that our moon was formed in a similar manner through the collision of two planetary embryos, an object about the size of Mars and Earth. The crash created a phenomenal amount of debris. Some of the debris created the moon and some orbited around the sun, Zuckerman said.

“The Earth certainly was put into accretion,” McKeegan said.

In the future, new techniques can be applied to determine the existence of such planets around old stars, such as our sun, or around young stars where planets are still in the process of formation, Zuckerman said.

“We have been working on the project for a couple of years,” he said. “In the coming years, astronomers will be attempting to determine how common is the existence of terrestrial orbit around the stars that are similar to the sun.”

UCLA’s results may well be the first observational evidence that terrestrial planets like these in our solar system are common, he said.

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