Now, two astronomers have shown that the dark night sky also tells us about the structure and formation of our solar system. In a recent paper published in the Astrophysical Journal Letters (547,L69) [], Scott J. Kenyon (Smithsonian Astrophysical Observatory) and Rogier A. Windhorst (Arizona State University) have used the dark night sky to set new limits on the amount of material in the outer reaches of the solar system. Their results tell us about the formation of planets like Pluto in the outer solar system.

In 1992, Jane Luu (Leiden Observatory) and Dave Jewitt (University of Hawaii) discovered the first solar system objects outside the orbits of Neptune. Using ground-based telescopes, Luu, Jewitt, and others have now discovered over one hundred Kuiper Belt objects, KBOs for short, in orbit around our Sun. The largest KBO has a diameter of nearly 2,000 kilometers; the smallest KBO is only about 100 km across.

Kenyon and Windhorst used the dark night sky to estimate the number of KBOs smaller than 100 km across. Each of these small KBOs is too faint for observations even with the largest telescopes on earth. All together, these KBOs produce enough diffuse light for astronomers to observe with instruments like the Hubble Space Telescope. The amount of diffuse light is related to the number of small KBOs; more KBOs reflect more light from the Sun. Small dust particles in the inner solar system produce a similar effect, the Zodiacal light, a cone-shaped glow near the Sun which can be observed with the naked eye shortly after sunset or shortly before sunrise.

The relative numbers of small and large KBOs is an important prediction of theories for planet formation in the outer solar system. Most theories of planetary formation begin with a thin circumstellar disk of gas and dust rotating around a newly-formed star. Planets like the Earth, Mars, and Jupiter grow from mergers of much smaller bodies, known as planetesimals, embedded in the disk. The planetesimals, ranging in size from rougly one meter to one kilometer across, collide almost continuously, careening off each other like pinballs in an arcade game. Eventually, enough of these small bodies stick together to make rocky planets like the Earth and Mars or icy bodies like Pluto. The slow growth process is similar to the proverbial snowball growing larger and larger as it rolls down a snowy slope.

As the larger objects grow even larger, they stir up and accelerate smaller bodies in the nebula. The grinding, shattering effect of constant high-speed collisions produces untold millions of micrometer-sized to meter-sized particles that reflect light from the Sun. In the outer solar system, this process should produce large KBOs which can be detected by telescopes on earth. The high speed collisions make small collision fragments which produce the diffuse light that Kenyon and Windhorst used to estimate the numbers of small KBOs.

Kenyon and Windhorst's result shows that the night sky is too dark for some models of planet formation. Surprisingly, the dark night sky is connected to the formation of the planet Neptune. If Neptune forms before the large KBOs, there are too few large KBOs and too many small KBOs. If Neptune forms at about the same time as the large KBOs, the brightness of the night sky is just right. In these models, most of small KBOs are the fragments of collisions between much larger objects.

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