The Outer Solar System

Science Goals / Science Portfolio / Dark Matter / Dark Energy / Solar System / Transient / Milky Way

Near-Earth Objects / Outer Solar System




The LSST has been identified as a national scientific priority in reports* by diverse national panels, including several National Academy of Sciences and federal agency advisory committees. Investigating the extent and content of the solar system beyond Neptune requires a detailed understanding of the Kuiper Belt, which in turn will lead to an improved understanding of the link between our Solar System and those being discovered around other stars. There is evidence for extensive material at large distances from central stars other than the Sun—in some cases this material extends to 1,100 AU from the star. The study of the outer solar system will not only clarify the formation history of our solar system, but will point the way to how other solar systems may form and how star formation in general proceeds.

The objects in the outer solar system are the most pristine material left from the protoplanetary cloud. This system of orbiting material may contain up to thirty trillion objects. One requires a sample of at least 10,000 objects in order to definitively sample both the spatial distribution and mass distribution. Telescope aperture and field-of-view cannot be traded for longer time on a smaller telescope because objects are faint and they move. In fact, most solar system objects discovered are lost because they are not monitored long enough to get a sufficiently precise orbit.

Finally, there is the potential for totally unexpected discovery in the outer solar system. Not only may we find new planets the size of Pluto or larger, we very likely will find objects with physical characteristics quite unlike those of the currently better known classes of solar system bodies.

We have only just discovered the vast, unexplored region of the solar system known as the Kuiper Belt. At the same time, we have now begun to image the dust and planetesimal debris disks around other stars in our search for planets around other stars. We have discovered close analogues to our own Kuiper Belt around some of these stars, for example, around the star Epsilon Eridani. These observations show arcs and local voids that may be due to the gravitational effects of embedded large planets. If we were to look at the solar system from afar, as we look out at other planetary disks today, we would see a similar void carved by the gravitational scattering of Neptune. In order to understand and interpret imaging and spectroscopy of planetary bodies around other stars, we need to understand the structure and composition of our own Kuiper Belt. In the coming decade, studies of extrasolar planetary systems will continue from new large telescopes on the ground and in Earth orbit. At the same time, the LSST will be able to determine the distribution of objects in the Kuiper Belt in great detail, which will enable comparison with the structure of extrasolar planetary disks.

Known objects in the outer solar system

Scientific questions to address with KBOs


  • The dynamical distribution of KBOs: further resonances? Reality of cutoff beyond 50AU? History of objects on high eccentricity/high inclination orbits?
  • The size distribution
  • The frequency of binarity and collision rate
  • Chemical composition via albedo, colors
  • Relationship to dust disks around other stars
  • Appropriate targets for future space missions?


How will LSST address these questions? LSST will search for TNOs over the entire footprint of the LSST survey to limiting magnitudes of r=24.5, finding approximately 30,000 objects larger than 100 km in diameter. TNOs move slowly, on the order of a few arcseconds per hour, thus observing a given field for about an hour and then using a digital shift-and-stack method will discover TNOs down to r=26.7 magnitudes. Observing the same field three times over two years will yield good orbits for almost all objects detected within the 9.6 square degree field of view. Observing a series of fields equally spaced along the ecliptic (+/- 20 degrees) will allow an indepth study of the dynamical structure of the Kuiper belt. If we spent 10% of the LSST observing time in this mode, we could discover and track roughly 35,000 faint TNOs.

* Quarks-to-Cosmos, Quantum Universe, Decadal Survey, Physics of the Universe, New Frontiers in Solar System Exploration