R. Paul Butler

Planets Orbiting Other Suns1,2

ABSTRACT After a century fraught with false claims, evidence for planets around other stars finally appears robust. Infrared imaging and spectroscopy of disks around stars foreshadow detailed models of the formation and evolution of planetary systems. Surveys of main‐sequence stars show that 5% harbor companions of (0.5–8)MJUP within 3 AU, peaked at lowest masses. Their orbits are either within 0.2 AU or eccentric, and occasionally both. These odd orbits suggest that dynamics with gas and planetesimals yield diverse systems and that stable, coplanar orbits of about nine giant and rocky planets may require special initial conditions. Far fewer stars (<1%) harbor (5–80)MJUP companions. This brown dwarf desert for companions stands in contrast to the abundant brown dwarfs that are freely floating.

The Planet around 51 Pegasi

Doppler measurements of 51 Pegasi have been made from 1995 October through 1996 August, with a precision of 5 m s-1. We find a period of 4.231 days, a velocity amplitude of 56 ± 1 m s-1, and a velocity curve that is essentially sinusoidal, all in excellent agreement with Mayor & Queloz. The only viable interpretation is a companion having minimum mass, m sin i = 0.45 MJupiter, in a circular orbit of radius of 0.051 AU, with an eccentricity less than 0.01. Alternative explanations involving stellar surface phenomena such as pulsation or spots are ruled out. The lack of tidal spin-up of the star constrains the mass of the companion to be less than 15 MJupiter. If the tidal Q-value is less than ~106 for the planet (close to Jupiters presumed value), then internal dissipation is adequate to circularize the orbit and synchronize the planets rotation. After subtracting the best-fit Keplerian velocity curve, the residuals exhibit no apparent variations at a level of 5 m s-1 during 10 months. The absence of further reflex motion along with limits from IR speckle observations rule out additional companions in a large portion of the parameter space of mass and orbital radius, including all masses greater than 1 MJupiter within 2.0 AU.

A Precision Velocity Study of Photometrically Stable Stars in the Cepheid Instability Strip

Results of a precision Doppler velocity survey of 15 stars that lie in or near the Cepheid instability strip are presented. Previous studies have shown that these stars are photometrically stable. Long-term radial velocity precision of 15 m s-1 has been achieved with the use of an iodine absorption cell and a high-resolution cross-dispersed echelle spectrometer. The stars show a variety of behavior from stability (at the level of 30 m s-1) to variability from 50 m s-1 to a few km s-1. Periodograms of many of the program stars show significant peaks at 50-80 days that are not associated with radial pulsation. Previously undetected binary companions have been found around two of the stars. Line profiles are compared to δ Cep.

Statistical Properties of Extrasolar Planets

The emerging statistical properties from the first 50 extrasolar planets are startlingly different from the picture that was imagined prior to 1995. About 0.75% of nearby solar type stars harbor jovian planets in 3 to 5 day circular orbits. Another ∽7% of stars have jupiter–mass companions orbiting in eccentric orbits within 3.5 AU. The mass distribution of substellar companions rises abruptly near 5 M Jup and continues increasing down to the detection limit near 1 M Jup -Orbital eccentricities correlate positively with semimajor axes, even for planets beyond the tidal circularization zone within 0.1 AU, distinguishing planets from binary stars. The planet bearing stars are metal–rich relative to both nearby stars and to the Sun. Analogs of Solar System planets have not been detected to date as they require precision of 3 m s −1 maintained for more than a decade.

First three planets

The first three extrasolar planets orbiting Solar-like stars have recently been discovered. All three were inferred from a periodicity in the optical Doppler measurements of their host stars, indicating a reflex motion in response to the gravitational force exerted by the planets. From such measurements and standard Newtonian physics, one may infer the orbital period and value of MPL sin i of 0.46, 2.5 and 6.5 MJUP, and orbital periods of 4.2, 117 and 1100 days, respectively. The most massive planet (around 70 Vir) has an eccentricity, e equals 0.38, larger than any in our Solar System, and one (around 51 Peg) has an orbital radius of 0.05 AU which is smaller than any in our Solar System. A general theory for the formation of planets must include these new characteristics.