Grains of Sand Grow Into Planets

This image shows the classic definition of our Solar System’s habitable zone, which is where the temperature is suitable for liquid water. Today, scientists also believe that additional habitats could exist beyond this traditionally defined region, such as in subsurface oceans on Jupiter’s moon Europa.
Credit: NASA

In a find that sheds light on how Earth-like planets may form, astronomers this week reported finding the first evidence of small, sandy particles orbiting a newborn solar system at about the same distance as the Earth orbits the Sun. The report will be published online in the journal Nature.

"Precisely how and when planets form is an open question," said study co-author Christopher Johns-Krull, assistant professor of physics and astronomy at Rice University. "We believe the disk-shaped clouds of dust around newly formed stars condense, forming microscopic grains of sand that eventually go on to become pebbles, boulders and whole planets."

In previous studies, astronomers have used infrared heat signals to identify microscopic dust particles around distant stars, but the method isn’t precise enough to tell astronomers just how big they become, and whether the particles orbit near the star, like the Earth does the Sun, or much further away at a distance more akin to Jupiter or Saturn.

This image shows the discovery of KH 15D as captured by the Wesleyan 0.6 m telescope. KH 15 D is a variable star found in the young cluster NGC 2264.
Credit: Wesleyan University/Van Vleck Observatory

In the new study, Johns-Krull and co-authors in the United States, Germany and Uzbekistan used reflected light from the sand itself to confirm the Earth-like orbit of grainy particles around a pair of stars called KH-15D in the constellation Monoceros. The stars are about 2,400 light years from Earth in the Cone Nebula, and they are only about 3 million years old, compared to the sun’s 4.5 billion years.

"We were attracted to this system because it appears bright and dim at different times, which is odd," Johns-Krull said.

The researchers found that the Earth has a nearly edge-on view of KH-15D. From this perspective, the disk blocks one of the stars from view, but its twin has an eccentric orbit that causes it to rise above the disk at regular intervals.

Data for this study was gathered over 12 years using a dozen observatories, including Hawaii’s W. M. Keck Observatory and Chile’s Very Large Telescope.
Credit: NASA / ESO

"These eclipses let us study the system with the star there and with the star effectively not there," Johns-Krull said. "It’s a very fortuitous arrangement because when the star is there all the time, it’s so bright that we can’t see the sand."

The team conducted both photometric and spectrographic analyses of data collected during the past 12 years from a dozen observatories, including the McDonald Observatory in west Texas, the Keck Observatory in Hawaii and the VLT on Mount Paranal in Chile.

"Because of how the light is being reflected there are opportunities to make observations about the chemical composition of these sand-like particles," said co-author William Herbst, an astronomer at Wesleyan University in Middletown, Conn. "That’s very exciting because it opens up so many doors for new type of research on this disk."

The research was funded by NASA and the Keck Foundation. Co-authors include Catrina Hamilton of Dickinson College; Katherine LeDuc of Wesleyan University; Joshua Winn of the Massachusetts Institute of Technology; Reinhard Mundt of the Max Planck Institute for Astronomy in Heidelberg, Germany; and Mansur Ibrahimov of the Ulugh Beg Astronomical Institute in Tashkent, Uzbekistan.

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