Dusty debris disks around main-sequence stars are signposts for the existence of planetesimals and exoplanets. From cross-correlating Hipparcos stars with the IRAS catalogs, we identify 146 stars within 120 pc of Earth that show excess emission at 60 µm. This search took special precautions to avoid false positives.Our sample is reasonably well distributed from late B to early K-type stars, but it contains very few later type stars. Even though IRAS flew more than 20 years ago and many astronomers have cross-correlated its catalogs with stellar catalogs, we were still able to newly identify debris disks at as many as 33 main-sequence stars; of these, 32 are within 100 pc of Earth. The power of an all-sky survey satellite like IRAS is evident when comparing our 33 new debris disks with the total of only 22 dusty debris disk stars detected first with the more sensitive, but pointed, satellite ISO . Our investigation focuses on the mass, dimensions, and evolution of dusty debris disks.

We propose 35 star systems within ∼70 pc of Earth as newly identified members of nearby young stellar kinematic groups; these identifications include the first A-and late-B type members of the AB Doradus moving group and field Argus Association. All but one of the 35 systems contain a bright solar-or earlier-type star that should make an excellent target for the next generation of adaptive optics (AO) imaging systems on large telescopes. AO imaging has revealed four massive planets in orbit around the λ Boo star HR 8799. Initially the planets were of uncertain mass due in large part to the uncertain age of the star. We find that HR 8799 is a likely member of the ∼30 Myr old Columba Association implying planet masses ∼6 times that of Jupiter. We consider Spitzer Space Telescope MIPS photometry of stars in the ∼30 Myr old Tucana/Horologium and Columba Associations, the ∼40 Myr old field Argus Association, and the ∼70 Myr old AB Doradus moving group. The percentage of stars in these young stellar groups that display excess emission above the stellar photosphere at 24 and 70 µm wavelengths -indicative of the presence of a dusty debris disk -is compared with corresponding percentages for members of 11 open clusters and stellar associations with ages between 8 and 750 Myr, thus elucidating the decay of debris disks with time.

From optical spectroscopic measurements we determine that the HD 15407 binary system is ∼80 Myr old. The primary, HD 15407A (spectral type F5 V), exhibits strong mid-infrared excess emission indicative of a recent catastrophic collision between rocky planetary embryos or planets in its inner planetary system. Synthesis of all known stars with large quantities of dust in their terrestrial planet zone indicates that for stars of roughly Solar mass this warm dust phenomenon occurs at ages between 30 and 100 Myr. In contrast, for stars of a few Solar masses, the dominant era of the final assembling of rocky planets occurs earlier, between 10 and 30 Myr age. The incidence of the warm dust phenomenon, when compared against models for the formation of rocky terrestrial-like bodies, implies that rocky planet formation in the terrestrial planet zone around Sun-like stars is common.-2architectures form around other stars. The study of extra-solar terrestrial planet formation must currently be carried out through indirect searches for the dusty aftermath of formation events. Since the dust is located in the terrestrial planet zone, it will have temperatures of ∼300 K and will emit most of its thermal radiation at ∼10 µm. Stars actively undergoing terrestrial planet formation will have substantial amounts of warm dust orbiting in their terrestrial planet zones and will thus show mid-infrared emission in excess of what one would expect from the star alone. Properly establishing the age of such mid-infrared excess systems is necessary for comparison with expectations from planet formation theories and simulations.We were alerted to excess mid-infrared emission around the star HD 15407A through a paper entitled "Detection of silica dust in the debris disk around HD 15407" presented at an October 2009 Spitzer meeting by H. Fujiwara (E. Becklin, personal communication 2009). At this meeting, the age for HD 15407A was suggested to be ∼2 Gyr. We felt this age could be in error, and initiated our own follow-up observations to independently constrain HD 15407A's age. Here we report these observations, our revised value for the age of the HD 15407 binary system, and a discussion of terrestrial planet zone collisions around adolescent Solar mass stars. ObservationsBoth components of the HD 15407 system were observed in cloudy conditions at Mauna Kea Observatory with the Keck I telescope and HIRES echelle spectrometer (Vogt et al. 1994). With the HIRES red collimator wavelengths between 4680 and 8700Å were covered at a resolution of 50,000 (as determined from the FWHM of single ThAr arclines). The final signal-to-noise ratio per pixel, measured at 6700Å, is 350 for HD 15407A and 200 for HD 15407B. Spectral data were reduced and extracted using both the MAKEE software package and standard IRAF tasks and echelle reduction procedures.

Stars form with gaseous and dusty circumstellar envelopes, which rapidly settle into disks that eventually give rise to planetary systems. Understanding the process by which these disks evolve is paramount in developing an accurate theory of planet formation that can account for the variety of planetary systems discovered so far. The formation of Earth-like planets through collisional accumulation of rocky objects within a disk has mainly been explored in theoretical and computational work in which post-collision ejecta evolution typically is ignored, although recent work has considered the fate of such material. Here we report observations of a young, Sun-like star (TYC 8241 2652 1) where infrared flux from post-collisional ejecta has decreased drastically, by a factor of about 30, over a period of less than two years. The star seems to have gone from hosting substantial quantities of dusty ejecta, in a region analogous to where the rocky planets orbit in the Solar System, to retaining at most a meagre amount of cooler dust. Such a phase of rapid ejecta evolution has not been previously predicted or observed, and no currently available physical model satisfactorily explains the observations.

Only a few solar-type main-sequence stars are known to be orbited by warm dust particles; the most extreme is the G0 field star BD +20 307 that emits $4% of its energy at mid-infrared wavelengths. We report the identification of a similarly dusty star HD 23514, an F6-type member of the Pleiades. A strong mid-IR silicate emission feature indicates the presence of small warm dust particles, but with the primary flux density peak at the nonstandard wavelength of $9 m. The existence of so much dust within an AU or so of these stars is not easily accounted for given the very brief lifetime in orbit of small particles. The apparent absence of very hot ( k1000 K ) dust at both stars suggests the possible presence of a planet closer to the stars than the dust. The observed frequency of the BD +20 307/ HD 23514 phenomenon indicates that the mass equivalent of Earth's Moon must be converted, via collisions of massive bodies, to tiny dust particles that find their way to the terrestrial planet zone during the first few hundred million years of the life of many (most?) Sun-like stars. Identification of these two dusty systems among youthful nearby solar-type stars suggests that terrestrial planet formation is common.