The Solar System Exploration Telescope (SSET)
by Alan Tokunaga
One of the hottest subjects in astronomy today is searching
for planets around other stars. Recent discoveries of planetary systems around
nearby stars have, for the first time, given us unequivocal evidence that solar
systems are common in the Universe. A new field of astronomical research has
developed to answer these questions:
- How many solar systems have Earth-like planets?
- How do solar systems, including our own, form?
- What is the likelihood of life in other solar systems?
These questions are not new, but we now have the technology
to obtain definitive answers.
An overexposed image of a star taken with
a conventional on-axis telescope. This scattered light will largely go
away with an off-axis telescope, enabling us to detect faint planets near
bright stars, and faint nebulosity surrounding young stars, where planets
may be forming.
A faint companion (TWA 5B) next to a bright
star. The estimated mass of the companion is 20 times the mass of Jupiter.
These objects are nearby-about 150 light-years from Earth. With a low
scattered light telescope, we will be able to observe planets as small
as Jupiter, but closer to a bright star. This will provide the means to
discover entirely new solar systems and measure their properties.
The proposed Solar Systems Exploration Telescope will enable
us to study the origin of our solar system by measuring the size and composition
of objects beyond the orbit of Pluto, and by observing asteroids that come close
to Earth. It will also be used to study existing solar systems around nearby
stars and solar systems now forming around very young stars.
During the last twenty years, there has been great progress
in the field of adaptive optics, which eliminates blurring by Earth's atmosphere.
These advances, as well as those in infrared detector technology, enable us
to develop the SSET. The SSET will be designed for maximum performance in the
infrared portion of the spectrum to take advantage of the high transmission,
excellent seeing, minimal water vapor, and low thermal background that characterize
the atmosphere above Mauna Kea.
In a conventional telescope (left), the optics
are "on-axis." In the alternative design we are considering
(right), the light comes to a focus off axis. This significantly reduces
the scattered light and heat emission from the telescope itself.
Infrared radiation is especially useful in measuring the temperature
and composition of astronomical bodies, particularly those obscured by dust
and gas in interstellar space. The infrared region is also the best place to
search for new planets, since they are cool and radiate nearly all their energy
in the infrared (as heat), rather than at optical wavelengths (as light). Fundamental
information about an object's luminosity, composition, and temperature can be
best obtained at infrared wavelengths.
Another exciting area of inquiry is the search for nebulae out
of which solar systems are likely to have formed or are in the process of forming.
To detect planets around these stars, we will need the low scattered light and
large aperture of the SSET. It is essential that the design reduce scattered
light in the telescope.
Jupiter as seen in the infrared
|The bright areas in this infrared image of Jupiter are
regions where heat is escaping through gaps in the clouds. Jupiter has an
internal heat source, and it emits twice as much heat as it receives from
the Sun. With the Solar Systems Exploration Telescope, we will be able to
obtain very high resolution images of Jupiter and its satellites, and to
acquire information about its composition as well. Massive planets ten to
twenty times the mass of Jupiter have been observed near our solar system
and around nearby stars.