Planet Characterization


When a planet passes between the observer and its host star, it blocks a small portion of the stellar surface, which dims the detected light by ~1%. The shape of the light curve can be used to measure the planet's radius (assuming the star's radius is known). Together with the planet's mass, this gives an average density of the planet.

I am working with the Transit Light Curve project, lead by Josh Winn and Matt Holmann, to obtain high-quality light curves of transiting planets identified by transit search teams such as the HAT consortium.

In this paper, UC grad student Peter Williams and I measured transit light curve for HAT-P-1 at Lick Observatory, along with several other groups from around the world:

The Transit Light Curve Project. VII. The Not-So-Bloated Exoplanet HAT-P-1b Winn, Holmann, Bakos, Pal, Johnson, Williams et al. 2007 AJ: ADS, astro-ph

As a team member in the Next 2000 Stars Consortium along with Debra Fischer and Bun'ei Sato, I was a part of the discovery of the transits of HD149026, which had an unusually large, rocky core:

The N2K Consortium. II. A Transiting Hot Saturn around HD 149026 with a Large Dense Core Sato, Fischer, ..., Johnson et al. 2005 ApJ: ADS, astro-ph

By blocking the surface of the star the transiting planet also alters the apparent radial velocity (RV) of the star. Out of transit, the rotating star has a constant RV, since the approaching (blue-shifted) and receding (red-shifted) portions of the surface contribute equally and cancel out (excluding the effects of star spots). However, when the planet traverses the surface of the star, parts of the rotating star's surface are blocked, causing an anomalous net Doppler shift, which changes the star's RV in a predictable manner known as the Rossiter-McLaughlin (RM) effect.

I am working with Josh Winn to observe the RM effect for transiting systems using in-transit RV measurements observed with Keck/HIRES and Subaru/HDS. By modeling the velocities we measure the projected angle between the star's spin axis and the planet's orbital axis. The spin-orbit alignment of exoplanets helps test models of planet migration, and also places our Solar System--with its well-aligned planets--in a broader context.

We recently published our measurement of spin-orbit alignment in the HAT-P-1 system:

Measurement of the Spin-Orbit Angle of Exoplanet HAT-P-1b

Johnson, Winn, Narita, et al. 2008 ApJ: ADS, astro-ph

The velocity time series shown at left is from our monitoring of HD189733 in this paper

Measurement of the Spin-Orbit Alignment in the Exoplanetary System HD 189733 Winn, Johnson, et al. 2006 ApJL: ADS, astro-ph

I was also a part of the follow-up TLC effort for HD189733:

The Transit Light Curve Project. V. System Parameters and Stellar Rotation Period of HD 189733 Winn, Holmann, Henry, ..., Johnson et al. 2007 AJ: ADS, astro-ph

Our other Rossiter modeling efforts include

Spin-Orbit Alignment for the Eccentric Exoplanet HD 147506b Winn, Johnson, et al. 2007 ApJ: ADS, astro-ph

Measurement of Spin-Orbit Alignment in an Extrasolar Planetary System Winn, ..., Johnson et al. 2005 ApJ: ADS, astro-ph

Planet Transits

The Rossiter-McLaughlin Effect

An artist's impression of a transiting Jupiter-mass planet. The yellow line shows the brightness as a function of time during the transit (click for a larger view, image from ESA).

Radial velocity as a function of time during the transit of HD189733 (from Winn, Johnson et al. 2006, ApJL)

The shape of the Rossiter-McLaughlin effect can be modeled to obtain the projected angle between the stellar spin axis and planetary orbital axis (figure from Gaudi & Winn 2006, ApJ)