Download Smallest Kuiper Belt Object Ever Detected

National Aeronautics and Space Administration
Smallest Kuiper Belt Object
Ever Detected
Taken from:
Hubble 2009: Science Year in Review
Produced by NASA Goddard Space Flight Center
and the Space Telescope Science Institute.
The full contents of this book include Hubble science articles, an overview of
the telescope, and more. The complete volume and its component sections are
available for download online at:
www.hubblesite.org/hubble_discoveries/science_year_in_review
HUBBLE 2009: SCIENCE YEAR IN REVIEW
HUBBLE SCIENCE: SMALLEST KUIPER BELT OBJECT EVER DETECTED
Smallest Kuiper Belt Object Ever Detected
The Kuiper belt, a region of small, icy bodies thought to be left over from the formation of the solar system, extends from
the orbit of Neptune to more than 5 trillion miles from the Sun. It contains numerous bodies called Kuiper belt objects
(KBOs), the most famous being Pluto and its moons.
Due to their small size, only the larger KBOs can be detected with direct imaging. Smaller objects, however, can be
detected indirectly when they pass in front of a background star and partially obscure the stellar light. These events,
called occultations, should last only several tenths of a second for such small and distant KBOs; therefore this method
of detection requires a dedicated observing plan that carefully monitors many stars. Hilke Schlichting, of the Canadian
Institute for Theoretical Astrophysics in Toronto, and her collaborators devised an unconventional way to detect these tiny
KBOs: by extracting their signatures from previously collected Hubble engineering data.
Hubble has three optical instruments called Fine Guidance Sensors (FGSs) that provide high-precision navigational
information to the space observatory’s attitude control system. They do this by exploiting the wave nature of light to make
extremely precise measurements of stellar positions. Schlichting and her team determined that the FGS instruments are
also sufficiently sensitive to detect the faint signal of a KBO passing in front of a star. Such an event would cause a brief
occultation and a diffraction signature in the FGS data as the light from the background guide star is bent around the
intervening foreground KBO.
The researchers selected 4.5 years of FGS observations for analysis. During this period Hubble’s FGSs spent a total of
12,000 hours observing along a strip of sky within 20 degrees of the solar system’s ecliptic plane. This is where scientists
expect to find the majority of KBOs.
Schlichting and her team analyzed the FGS observations of 50,000 guide stars in total. Scouring the huge database,
they found a single 0.3-second-long occultation event. Detection was only possible because the FGS instruments sample
changes in starlight 40 times a second. The researchers found the KBO at an inclination of 14 degrees to the ecliptic, and
This artist’s impression portrays the tiny Kuiper belt object (KBO) that was detected by Hubble. The icy relic from the early solar system
is just over half a mile across—too small for Hubble to photograph. The object was detected when it temporarily disrupted the starlight
as it passed in front of a background star.
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assumed the KBO was in a circular orbit. The KBO’s distance was then estimated from the duration of the occultation, and
the amount of dimming was used to calculate the size of the object.
The tiny body appears to be just over a half mile—or approximately 1 kilometer—across and is located 4.2 billion miles
away. The object is so small and distant that it is 100 times dimmer in reflected sunlight than what Hubble can see directly.
In fact, the smallest KBO previously seen in reflected light by any telescope is roughly 30 miles across, or 60 times larger.
This small KBO represents a missing link between micron-sized dust particles and larger KBOs in the Pluto-size class of
1,000 km. Researchers have long suspected that these bodies exist, but previously lacked proof for objects of this size.
They are believed to be the result of collisions among the members of the original Kuiper belt population.
The detection of only one event actually reveals a deficit of sub-kilometer-sized objects compared with the population extrapolated
to exist from the known number of large Kuiper belt objects. This implies that sub-kilometer-sized objects are undergoing
collisional erosion as well. These small bodies are likely the source material for comets, and additional observations may shed
light on whether comets are solid, like pieces of rock, or more closely resemble collections of rubble held together by gravity.
In an effort to uncover more small KBOs, the team plans to analyze the remaining FGS data for nearly the full duration
of Hubble operations since its launch in 1990. The discovery of additional objects could provide direct evidence for the
theorized population of approximately 10,000 bodies thought to supply the number of observed Jupiter-family comets—
those with periods of less than 20 years. Such research is a powerful illustration of the capability of archived Hubble data
to enable new and important discoveries.
An artist’s illustration of the Kuiper belt, a vast
ring of icy debris encircling the outer rim of
the solar system beginning just beyond
Neptune.
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A brief occultation and diffraction signature was seen in the Hubble Fine Guidance Sensor data as the light from a background star was bent around
the intervening foreground KBO. (Artist’s concept).
Further Reading
Cowen, R., “On the Fringe,” Science News, 177(2), 16–21, 2010.
Roques, F., G. Georgevits, and A. Doressoundiram, “The Kuiper Belt Explored by Serendipitous Stellar Occultations,” in The Solar System Beyond Neptune, M.A. Barucci, H. Boehnhardt, D.P. Cruikshank, and A. Morbidelli (eds.), Tucson, AZ: University of Arizona Press,
545–556, 2008.
Schlichting, H., et al. “A Single Sub-kilometre Kuiper Belt Object from a Stellar Occultation in Archival Data,” Nature, 462, 895–897, 2009.
Dr. Hilke Schlichting is a postdoctoral fellow at the Canadian Institute for Theoretical Astrophysics in
Toronto. Her work focuses on planet formation, particularly the dynamical and physical properties of
the Kuiper belt and Oort cloud populations, and the information they reveal about the formation of the
solar system. Her past research has included the origins of planetary spins and the velocity distribution
of planetestimals and protoplanets during various stages of their formation. Schlichting received her
B.A. and M.S. in physics from the University of Cambridge and her Ph.D. in astrophysics from the California Institute of Technology. She was born in Bad Soden, Germany, and has lived in Germany, Japan,
England, and the United States.
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