prepublication copy - The Department of Astronomy & Astrophysics ...

us was certainly evident during the past decade. Here we list just a few of the most far-reaching
examples.
The surprising discovery in 1998 that the expansion of the universe is accelerating rather than
slowing, due to the repulsive gravity of dark energy, has changed the way we think about the evolution
and destiny of the universe and has challenged our understanding of physics at the most fundamental
level. In the coming decade, an optimized and coordinated set of facilities on the ground and in space
will test whether the simplest hypothesis-dark energy is the quantum energy of the vacuum⎯is the
correct explanation or if something more exotic is needed—as must be the case for the inflationary epoch,
an earlier period of acceleration. It is even possible that we will need a modification of Einstein’s general
relativity. Either way, the implications for both astronomy and physics are profound.
Telescopes are time machines: because light travels across the cosmos at a finite speed, the most
distant objects probe the furthest back in time. The 13.7 billion year old cosmic microwave background is
seen in the millimeter band. The latest record holder (early 2010) for the most distant object is a gammaray
burst that occurred 13.1 billion years ago when the universe was 0.6 billion years old. Detected by a
NASA Explorer Program satellite called Swift, its distance was measured by follow-up observations from
telescopes on the ground. In the coming decade, powerful new observatories on the ground and in space
will allow us to push back to still earlier times and glimpse the end of the cosmic dark ages signaled by
the formation of the first-ever luminous sources in the universe⎯the first generation of stars.
Closer to home, the last decade has seen the discovery of well over four hundred planets orbiting
nearby stars. While the existence of extra-solar planets had long been anticipated, the astonishing
discovery is that the planets and their orbits seem to be nothing like our own. In the coming decade, new
facilities on the ground and in space will enable us to detect potentially life-bearing planets similar to the
Earth.
Looking forward, the most promising areas for revolutionary discoveries are highlighted in the
following subsections. This is indeed a special time in history. The unexpected can be expected with
confidence.
The Discovery of Habitable Planets
We are rapidly building our knowledge of nearby analogs to our own Solar System’s planets,
most recently with the launch of NASA’s Kepler mission. The salient feature of the planetary menagerie
of which we are currently aware is its diversity⎯in every measureable sense⎯of the properties of the
planets as well as the properties of the stars around which they are found. We are also improving our
understanding of the planet formation process, and ALMA is expected to unveil the birthing of new
worlds.
Detection methods till now have only been able to discover massive planets rivaling the giants in
our Solar System (Figure 2-1 upper) or larger (Figure 2-1 lower). The most profound discovery in the
coming decade may be the detection of potentially habitable Earth-like planets orbiting other stars. To
find evidence that life exists beyond our Earth is a longstanding dream of humanity, and it is now coming
within our reach.
The search for life around other stars is a multi-stage process. Although JWST may be able to
take the first steps, more complex and specialized instrumentation is also needed, requiring a longer-term
program. First, the frequency with which Earth-sized planets occur in zones around stars where liquids
such as water are stable on planetary surfaces must be measured (see Box 2-1). Stars will then be targeted
that are sufficiently close to us that the light of the companion planets can be separated from the glare of
the parent star and studied in great detail; this will allow us to find signatures of molecules that indicate a
potentially habitable environment. Here, the opportunities are suddenly bountiful, as we have understood
over this past decade that, for example, stars much lower in mass than our Sun may have orbiting
habitable planets that are much easier to spot. Thus, the plan for the coming decade is to perform the
necessary target reconnaissance surveys to inform next-generation mission designs while simultaneously
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
2-2