Search

We present precision radial velocity data that reveal a multiple exoplanet system orbiting the bright nearby G5V star 61 Virginis. Our 4.6 years of combined Keck/HIRES and Anglo-Australian Telescope precision radial velocities indicate the hitherto unknown presence of at least three planets orbiting this well-studied star. These planets are all on low-eccentricity orbits with periods of 4.2, 38.0, and 124.0 days, and projected masses (Msin i) of 5.1, 18.2, and 24.0 M [?], respectively. Test integrations of systems consistent with the radial velocity data suggest that the configuration is dynamically stable. Depending on the effectiveness of tidal dissipation within the inner planet, the inner two planets may have evolved into an eccentricity fixed-point configuration in which the apsidal lines of all three planets corotate. This conjecture can be tested with additional observations. We present a 16-year time series of photometric observations of 61 Virginis, which comprise 1194 individual measurements, and indicate that it has excellent photometric stability. No significant photometric variations at the periods of the proposed planets have been detected. This new system is the first known example of a G-type Sun-like star hosting a Super-Earth mass planet. It joins HD 75732 (55 Cnc), HD 69830, GJ 581, HD 40307, and GJ 876 in a growing group of exoplanet systems that have multiple planets orbiting with periods less than an Earth-year. The ubiquity of such systems portends that space-based transit-search missions such as Kepler and CoRoT will find many multi-transiting systems.

Precision Doppler measurements from an intensive 48 night "Rocky Planet Search" observing campaign on the Anglo-Australian Telescope (AAT) have revealed the presence of a low-mass exoplanet orbiting the G1 dwarf HD 16417. Subsequent Doppler observations with the AAT, as well as independent observations obtained by the Keck Planet Search, have confirmed this initial detection and refine the orbital parameters to period 17.24 +- 0.01 d, eccentricity 0.20 +- 0.09, orbital semimajor axis 0.14 +- 0.01 AU, and minimum planet mass 22.1 +- 2.0 M Earth. HD 16417 raises the number of published exoplanets with minimum masses of less than 25 M Earth to 18. Interestingly, the distribution of detected sub-25 M Earth planets over the spectral types G, K, and M is almost uniform. The detection of HD 16417b by an intensive observing campaign clearly demonstrates the need for extended and contiguous observing campaigns when aiming to detect low-amplitude Doppler planets in short-period orbits. Perhaps most critically it demonstrates that the search for low-mass Doppler planets will eventually require these traditional "bright-time" projects to extend throughout dark lunations.

Precision Doppler velocity measurements from the Anglo-Australian Telescope reveal a planet with a 9.4 +- 0.4 year period orbiting the M1.5 dwarf GJ 832. Within measurement uncertainty the orbit is circular, and the minimum mass (msin i) of the planet is 0.64 +- 0.06 M JUP. GJ 832 appears to be depleted in metals by at least 50% relative to the Sun, as are a significant fraction of the M dwarfs known to host exoplanets. GJ 832 adds another Jupiter-mass planet to the known census of M dwarf exoplanets, which currently includes a significant number of Neptune-mass planets. GJ 832 is an excellent candidate for astrometric orbit determination with asin i = 0.95 mas. GJ 832b has the second largest angular distance from its star among radial velocity detected exoplanets (0.69 arcsec) making it a potentially interesting target for future direct detection.

We report the detection of a second exoplanet orbiting the G6 V dwarf HD 73526. This second planet has an orbital period of 377 days, putting it in a 2 : 1 resonance with the previously known exoplanet, the orbital period for which is updated to 188 days. Dynamical modeling of the combined system allows solution for a self-consistent set of orbital elements for both components. HD 73526 is the fourth exoplanetary system (of a total of 18 systems with two or more components currently known) to have components detected in 2 : 1 resonance. Finding such a large fraction of multiple planets (more than 20%) in 2 : 1 resonance strongly suggests that orbital migration, halted by stabilization in a trapping resonance, plays an important role in the evolution of exoplanets in multiple planet systems.

We report the detection of three new low-mass planets from the Anglo-Australian Planet Search. The three parent stars of these planets are chromospherically quiet main-sequence G dwarfs with metallicities ranging from roughly solar (HD 117618 and HD 208487) to metal enriched (HD 102117). The orbital periods range from 20.8 to 130 days, the minimum masses from roughly 0.5MSat to 0.5MJup, and the eccentricities from 0.08 to 0.37, with the planet in the smallest orbit (HD 102117) having the smallest eccentricity. With semiamplitudes of 10.6-19 m s-1, these planets induce Doppler amplitudes similar to those of Jupiter analogs, albeit with shorter periods. Many of the most interesting future Doppler planets will be detected at these semiamplitude levels, placing a premium on measurement precision. The detection of such amplitudes in data extending back 6 yr gives confidence in the Anglo-Australian Planet Search's ability to detect Jupiter analogs as our time baseline extends to 12 yr. We discuss the criticality of such detections for the design of the next generation of extremely large telescopes and also highlight prospects for suitable observing strategies to push to below 1 m s-1 precisions for bright stars in a search for sub-Neptunian planets.

We report the detection of four new extrasolar planets from the Anglo-Australian Planet Search orbiting the metal-enriched stars HD 73526, HD 76700, HD 30177, and HD 2039. The planetary companion of HD 76700 has a circular orbit with a period of 3.98 days. With M sin i = (0.197 +- 0.017)MJ, or 0.69MSAT, it is one of the lowest minimum mass extrasolar planets yet detected. The remaining planets all have elliptical orbits with periods ranging from 190.5 days to 4.4 yr. All four planets have been found orbiting hosts from a subsample of 20 metal-enriched and faint (V < 9) stars, which was added to the Anglo-Australian Planet Search's magnitude-limited V < 7.5 main sample in 1999 October. These stars were selected for metal enrichment on the basis of their Stromgren photometry, and their enrichment has been subsequently confirmed by detailed spectroscopic analysis.

We report the detection of two new extrasolar planets from the Anglo-Australian Planet Search around the stars HD 142 and HD 23079. The planet orbiting HD 142 has an orbital period of just under 1 yr, while that orbiting HD 23079 has a period of just under 2 yr. HD 142 falls into the class of "eccentric" gas giants. HD 23079 joins the group of "[?] Ret-like" gas giant planets, with near-circular orbits outside a radius of 0.1 AU. The recent discovery of several more members of this class provides new impetus for the extension of existing planet searches to longer periods in the search for Jupiter-like planets in Jupiter-like orbits.

We have detected the Doppler signature of a gas-giant exoplanet orbiting the star HD 38283, in an eccentric orbit with a period of almost exactly one year (P = 363.2 +- 1.6 d, m sin i = 0.34 +- 0.02 M Jup, e = 0.41 +- 0.16). The detection of a planet with period very close to one year critically relied on year-round observation of this circumpolar star. Discovering a planet in a 1 AU orbit around a G dwarf star has prompted us to look more closely at the question of the habitability of the satellites of such planets. Regular satellites orbit all the giant planets in our solar system, suggesting that their formation is a natural by-product of the planet formation process. There is no reason for exomoon formation not to be similarly likely in exoplanetary systems. Moreover, our current understanding of that formation process does not preclude satellite formation in systems where gas giants undergo migration from their formation locations into the terrestrial planet habitable zone. Indeed, regular satellite formation and Type II migration are both linked to the clearing of a gap in the protoplanetary disk by a planet, and so may be inextricably linked. Migration would also multiply the chances of capturing both irregular satellites and Trojan companions sufficiently massive to be habitable. The habitability of such exomoons and exo-Trojans will critically depend on their mass, whether or not they host a magnetosphere, and (for the exomoon case) their orbital radius around the host exoplanet.

We present 12 years of precision Doppler data for the very nearby G3 star HD 102365, which reveals the presence of a Neptune-like planet with a 16.0 M Earth minimum mass in a 122.1 day orbit. Very few "Super Earth" planets have been discovered to date in orbits this large and those that have been found reside in multiple systems of between three and six planets. HD 102365 b, in contrast, appears to orbit its star in splendid isolation. Analysis of the residuals to our Keplerian fit for HD 102365 b indicates that there are no other planets with minimum mass above 0.3 M Jup orbiting within 5 AU and no other "Super Earths" more massive than 10 M Earth orbiting at periods shorter than 50 days. At periods of less than 20 days these limits drop to as low as 6 M Earth. There are now 32 exoplanets known with minimum mass below 20 M Earth, and interestingly the period distributions of these low-mass planets seem to be similar whether they orbit M-, K-, or G-type dwarfs.