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FEPS Asks 'Who's the Oddball?'

Artist Concept
An artist concept shows a planet-forming disk around a young star.
Credit: NASA/JPL-Caltech/T. Pyle (SSC)

Is our solar system a universal oddball? Or, is it just one of many?

By determining whether our solar system is an oddball, we can begin to infer whether Earth-like planets and life are rare in the universe. Armed with the super-sensitive infrared eyes of NASA's Spitzer Space Telescope, astronomers on the Formation & Evolution of Planetary Systems (FEPS) Legacy team are looking for dusty clues that may help answer these questions.

Currently, astronomers know of only one star and solar system with a planet where the conditions are conducive to life. That star is our Sun, and the life-sustaining planet is Earth. Thus, in their quest to determine whether we are universal oddballs, FEPS scientists are studying 330 Sun-like stars (stars about the size of the Sun) in our Milky Way galaxy. Their search begins with looking at disks of material around these stars.

"Spitzer's unprecedented infrared sensitivity has presented us with a great opportunity to study the disks of debris around stars like our Sun, and determine if there are asteroid and Kuiper belt analogues, and possibly even planets around them," said FEPS Principal Investigator, Dr. Michael Meyer.

Clues in the Dust

Limited by current technology, astronomers cannot resolve individual planets in distant solar systems. So how do FEPS astronomers learn about what may be lurking around these stars? The answer is in the dust. By studying the behavior of dust around a distant star, astronomers can infer whether there are planets and rocky belts orbiting the star, or if the system is conducive to life.

Just as students sitting at the back of a classroom cannot see the small piece of chalk in their teacher's hand, astronomers from Earth cannot directly observe planets and asteroid belts in other solar systems. They are just too far away. But when the teacher grinds the chalk stick on a board and spreads its dust to relay a lesson, even students sitting tens of feet away can see that their instructor is using chalk.

Similarly, it is easier for astronomers to see dust around a star than it is for them to detect rocks or planets. Like chalk dust on the classroom board, dust around a distant star covers more area than a planet, comet, or asteroid. As the small dust particles absorb heat from the host star, they re-emit most of their light at long infrared wavelengths, which Spitzer can detect.

The dust closest to the star is the warmest and dust particles get progressively cooler toward the outer edges of the solar system. An even distribution of energy indicates to astronomers that the dust disk is continuous. If the dust is all one temperature, scientists know that it is concentrated at a certain distance, and can infer the presence of an asteroid or Kuiper belt. Any gaps in the disk that could indicate a planet and will show up as a "dip" in astronomical graphs made with Spitzer data.

According to FEPS Co-Investigator Dr. Hillenbrand, Spitzer is not the first infrared telescope to use this technique for detecting planets, but "its spectacular sensitivity and spatial resolution helps astronomers study disks around stars that no previous infrared telescope has been able to detect."

FEPS Co-Investigator Dr. Amaya Moro-Martin notes that a dusty disk could also hold clues about whether the solar system is conducive to sustaining life.

"A dusty disk indicates a chaotic environment where rocks are constantly crashing into each other. On Earth we've been able to discern several epochs of mass extinction probably caused by impact collisionsÉthe most recent one was the dinosaurs' and some before that were much worse," she said.

Stars Across the Ages

In addition to being "Sun-like," the FEPS sample of 330 stars is special because they range in age from 3 million to 3 billion years old. Our Sun is approximately five billion years old. By observing Sun-like stars of different ages, astronomers hope to gain some insights into the development of our own solar system.

Since solar systems like our own take billions of years to form, there is no way for astronomers to watch an entire system develop. Instead, scientists have to piece together their development stages through snapshots.

To explain the process, some astronomers use the imaginary analogy of an alien visiting Earth for 20 minutes. In this example, if the alien wanted to learn about the life cycle of humans, the visitor couldn't do it by watching one human for 20 minutes. Instead, our cosmic explorer would have to take snapshots of lots of humans at different stages of development; maybe a pregnant mother, a baby, a teenager, and a senior adult. Then once arriving back home, the alien would piece together the life cycle of a human based on its pictures and observations.

From snapshots of space, astronomers know that solar systems form from disks of leftover dust and gas that swirl around stars after they stop growing, or accreting mass. This is known as a dust disk.

Eventually the material in the dust disk will collect to build rocky icy objects like comets, asteroids, and terrestrial planets. Remaining gas may collect around the largest rocky cores to form gas giant planets like Jupiter and Neptune. This new disk, which includes planets and rock belts, is called a debris disk. Our solar system is a debris disk.

According to Moro-Martin, 650 million years after the terrestrial planets formed in our solar system, they were subject to the 'Late Heavy Bombardment,' an evolutionary phase that resurfaced the Moon and the terrestrial planets, creating the lunar basins and leaving numerous impact craters that can still be seen on the Moon, Mercury, and Mars.

"If you were an outside observer studying our young Sun during this time, you'd see a very bright and dusty debris disk. With FEPS, we are looking for dust around stars with ages similar to our young Sun during its tumultuous bombardment phase. We want to find out how many of these stars are surrounded by bright debris disks and may be traveling the same evolutionary paths our Sun did," she said.

FEPS Legacy

Only a few years into their project, FEPS scientists have already detected potential planetary systems around approximately 10 to 20 percent of stars in their sample. The results indicate that solar systems may be more common and resilient than previously thought. Hillenbrand attributes this success to Spitzer's ability to detect holes in debris disks.

"FEPS is largely a discovery project. We are basically identifying solar systems with potential belts and planets for future missions to observe more closely," said Hillenbrand.

According to FEPS Co-Investigator Dr. Steve Strom, the project will also leave a legacy as one of the first projects to offer insights on "what types of disks lead to what types of solar systems." In other words, by studying the disks around very young (around 3 million years old) Sun-like stars and relatively older (around 3 billion years old) stars in the same project, scientists can see how the diversity of initial disk conditions can develop into different types of solar systems.

"FEPS' legacy will inspire observations for decades to come, if not longer," said Strom.



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