Only thirteen years ago we knew of one star that harbored a gas giant planet: our own Sun. Since then planet hunters have discovered more than 265 stars that have one or more planetary companions. The occurrence rates, system architectures and physical properties of these distant worlds shape our understanding of planet formation in general, and the origins of the Solar System in particular.

Most known planets have been detected using precision the Doppler technique, by which the reflex motion of a star due to the gravitational tug of planet is detected (see movie to right). And the majority of planet-harboring stars are similar to the Sun, with masses between 0.8 and 1.2 solar masses. Low-mass M dwarfs (red dwarfs) are too faint to observe with all but the largest telescopes. At the other end of the mass scale, massive A and F stars rotate quickly and pulsate, which limits our ability to detect the stellar wobble induced by a planet.

Measuring the occurrence rate and properties of planets as a function of stellar mass provides an important test of planet formation theories. For example, in the core accretion planet formation model, higher mass stars should be more likely to harbor a Jovian planet because their massive disks contain a higher surface density of solid materials that serve as the building blocks for planets. To test this prediction I am working with the California Planet Search consortium to search for planets around stars of a wide range of masses using some of the worlds largest optical telescopes.

The problems inherent to high-mass stars can be circumvented by observing them after they have evolved off of the main sequence onto the subgiant branch. The "retired" A and F stars are much more amenable to precision Doppler measurements than their main-sequence counterparts. We describe our sample of subgiants and the detection of a short-period planet in

An Eccentric Hot Jupiter Orbiting the Subgiant HD 185269 Johnson, Marcy, Fischer et al. 2006 ApJ: ADS, astro-ph

and our results are summarized in my recent review paper

Planets Around Massive Subgiants

Johnson 2008, ASP Conf Proc., V398, p59: astro-ph

In two subsequent papers we announced 5 new planets orbiting very massive stars:

Retired A Stars and Their Companions: Exoplanets Orbiting Three Intermediate-Mass Subgiants Johnson, Fischer, Marcy et al. 2007 ApJ: ADS, astro-ph

Retired A Stars and Their Companions. II. Jovian planets orbiting κ CrB and HD 167042 Johnson, Marcy, Fischer et al. 2008 ApJ: ADS, astro-ph

The Relationship Between

Stellar Mass and Exoplanets

The intrinsic faintness of M stars can be overcome by using the 10-meter Keck telescope. We describe the NASA Keck M Dwarfs Planet Search here:

A Long-Period Jupiter-Mass Planet Orbiting the Nearby M Dwarf GJ 849 Butler, Johnson, Marcy et al. 2006, PASP: ADS, astro-ph

We announced another low-mass planet host star (GL317), along with the results of my study of planet occurrence with stellar mass here:

A New Planet Around an M Dwarf: Revealing a Correlation Between Exoplanets and Stellar Mass Johnson, Butler, Marcy et al. 2006 ApJ: ADS, astro-ph

In collaboration with Debra Fischer and the Next 2000 Stars planet search team, I describe a modification to the Butler et al (1996) Doppler measurement technique, along with 3 new short-period planets:

The N2K Consortium. VI. Doppler Shifts without Templates and Three New Short-Period Planets Johnson, Fischer, Marcy, et al. (2006) ApJ, astro-ph

Figure illustrating the rising occurrence rate of Jovian planets as a function of stellar mass from Johnson, Butler, Marcy et al. 2007, ApJ (click for larger view).

Movie illustrating the reflex motion of a star in response to the gravitational tug of an orbiting planet. The planet causes the star to wobble, periodically changing its radial velocity.