TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 3
the circumstellar disk, or are they launched by magnetic fields entrained or dynamo-amplified in the disk
itself Detection of molecular bands from simple molecules (as well as PAH and silicate features) will
enable the characterization of chemical and physical gradients in disks.
Darwin/TPF-I will be especially sensitive to young planets, which tend to be larger, hotter, and therefore
brighter than mature objects. Forming gas and ice giants will be easy to detect. Even forming or young,
rocky terrestrial planets are expected to be brighter, especially following recent accretion events or
impacts.
Observations of more mature planetary systems and debris disks with ages ranging from a few million to
several hundred million years will lead to direct tests of planetary system evolution models. The
Darwin/TPF-I planet-finding capability will enable the direct detection of forming and evolving planets.
Large impacts on rocky planets are expected to produce global “lava oceans” that will glow at 10 μm for
thousands of years. Thus, direct observations of the conditions following giant impacts and equivalents
of the “Late-Phase Heavy Bombardment” suspected to have occurred about 800 Myr after the birth of our
own Solar System may be observable in other planetary systems. Source selection would rely on the
detection of extensive debris disks that may indicate a high rate of collisions.
The Darwin/TPF-I observations of forming and evolving planetary systems will complement the prime
mission of planet detection and characterization by providing direct tests of planet formation and
evolution models.
3.2.2 Stellar and Planetary Death and Cosmic Recycling
As stars of all masses evolve off the main sequence, they develop cool, extended envelopes that reprocess
most of the emitted starlight into infrared radiation observable with TPF-I. Low- and intermediate-mass
stars (M = 0.8 − 8 M) make up more than 90 per cent of all the stars that have died in the Universe up to
the present time. At the end of their main sequence life-time, they enter the high-luminosity asymptotic
giant branch (AGB) phase. During the short so-called superwind phase, the stars eject their hydrogen-rich
outer layers to reveal the chemically enriched deeper layers of the star. These stars obtain the highest
luminosity and the largest diameter in their existence with the size or nearly an AU. AGB stars are
expected to either vaporize, or swallow their planetary systems. Emission from silicates, silicon carbide
(SiC), PAHs, and some ices will provide powerful probes of physical and chemical evolution of these
objects. In the closest AGB stars, TPF-I/Darwin will resolve the photospheres; in more distant objects, it
will probe their winds and dying planetary systems in great details.
After this final burst of activity, these stars evolve into hot, compact white dwarfs with masses in the
range of 0.6−1.4 M o . The expanding ejecta surrounding the star becomes ionized and forms a planetary
nebula before dispersing into the interstellar medium (ISM).
Recently, the Spitzer Space Telescope had detected infrared excess emission from about 15 to 20% of old
white dwarf stars near the Sun (Reach et al. 2006; Mullally et al. 2006). This emission indicates the
presence of debris disks consisting of mostly large-solid particles that have resisted being dragged into the
central white dwarf by the Poynting-Robertson drag for the age of the white dwarf. Such debris disks
surrounding aging white dwarfs may trace the remnants of planetary systems that were destroyed during
the post-main-sequence red-giant phase of their parent stars. There are hints that in-spiraling solids may
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