Disk resembling asteroid belt found around nearby star
Posted: December 30, 2001

  Solar system
zeta Leporis
A comparison of our solar system's asteroid belt (top) and the zeta Leporis system (bottom). Credit: adapted from UCLA, Department of Physics and Astronomy: C.H. Chen and M.Jura
When unusually warm dust was first discovered (1991) around a nearby star, called zeta Leporis, infrared astronomers begun hunting in detail for the heat source. According to the latest research at UCLA, what the star may be undergoing is asteroid and planet formation similar to that of our own early solar system. For infrared astronomers the warm particle halo may reveal more than just a hot cloud. It may reveal a dusty disk that resembles an asteroid belt.

Michael Jura and Catherine Chen reported their most recent findings at the annual meeting of the American Astronomical Society.

"We chose to study zeta Leporis because it was known from IRAS (the Infrared Astronomy satellite) to display an infrared excess of unusually warm grains", says Jura. "Also, it is very nearby." Located in the constellation Lepus (the Hare) the asteroid candidate system is about 70 light years from our sun. Although it is almost twice as massive as our sun, zeta Leporis is only about 100 million years old. This is young in astronomical terms compared to our sun, which is approximately 4.5 billion years old.

Chen and Jura found that the tiny particles are heated on average to a toasty 350 degrees Kelvin (77 degrees Celsius, or 170 degrees Fahrenheit), which in turn reveals their distance from the star. They estimate that the disk -- with a mass comparable to Earth's -- contains about 1,000 times more material than our own asteroid belt.

What has excited and surprised the astronomers is that the dust shouldn't be there. The dusty disk indeed is so close to its parent star that the particle halo should have disappeared long ago as a fleeting and inward-sprialing tail. In less than 20,000 years the disk would have spiraled into zeta Leporis unless there was some source regenerating it. But the persistent warm band persists within a mere 6 astronomical units (AU) of its star. (One AU is the distance between the Sun and the Earth, roughly 150 million kilometers.) By comparison, our solar system's asteroid belt is 2.7 AU from the Sun.

Young stars like zeta Leporis also likely wouldn't have sufficient time to form solid bodies, which could collide to scatter dust as in our own zodiacal light seen near the asteroid belt. But on zeta Leporis, the constant replenishing of the dusty disk might be a case of asteroids that, according to Jura, "appear to be colliding violently with each other".

We can put the zeta Leporis discovery in perspective for our own solar system. The analog would be similar to when our Sun was less than 100 million years old and its then massive asteroid belt collided and scattered again to reach its current reduced dimensions. By looking at very young stars, the researchers get a glimpse looking back towards how our own solar system progressed from dust accretion to form planets and asteroids.

"The value of this work in terms of understanding the history and evolution of our own Solar System is that we have good evidence for the formation of large rocky bodies. These are strongly suspected to be formed, but there is no direct evidence", says Jura. "To date, all the planets that have been found around main sequence stars are similar in mass -- and presumably in composition-- to Jupiter. The Earth has a 'rocky' composition. To date, no terrestrial type planets have been identified. Our work shows that at least some kinds of large-ish asteroid bodies have been formed elsewhere."

But the huge, dusty disk on young zeta Leporis may yet offer even more surprises as its chemical composition is analyzed.

"Our current findings may be just the tip of the iceberg of what we may ultimately learn about the objects surrounding zeta Leporis," Chen said.

Ultimately the hunt is for nearby planets to study. By identifying what may be a galactic replay of how our own solar system was formed, Jura and Chen have found evidence of a massive asteroid belt around a nearby star. These findings could indicate that such planets are forming there or have already formed.

"In simplest terms, our planets formed when smaller objects smashed together," she said. "Dust that surrounds a star will eventually either fall into the star, or collide with itself and create bigger particles. The particles we can identify around zeta Leporis may be forming chunks of rock or larger objects; asteroids or even planets may be forming or have already formed around zeta Leporis."

What's next
Since the discovery that the dust around zeta Leporis is unusually warm was first published in 1991 by astronomers Hartmut Aumann and Ronald Probst, the main challenges to doing further planet formation studies has been the background heat emitted from Earth.

"A major problem with doing IR work from the ground is that the atmosphere and the telescope emit a lot of infrared radiation," says Jura. "We are always fighting the background. This is a major reason for launching

"We hope to obtain infrared spectra of the emission from zeta Leporis," Chen said. "We want to know if the asteroids around this star are similar in composition to objects in our solar system, and we want to learn if the processes we now see unfolding on zeta Leporis can help us understand how the planets in our own solar system formed."

Future candidate searches for terrestrial planets will depend on advanced combinations of instrument engineering and new analytical methods. In addition to being detected by the ground-based Long Wavelength Spectrometer (an infrared camera on the 10-meter telescope at the Keck Observatory on Mauna Kea, Hawaii) zeta Leporis itself was first discovered in 1983 with the Infrared Astronomy satellite (IRAS). Future space-based infrared telescopes such as SIRTF promise much higher resolution.

Chen concludes: "The next step is to get an infrared spectrum of this area [zeta Leporis], which would give us an indication of their composition."