John Johnson

John Johnson

Exoplanet detection and characterization

John Johnson

As recently as 1995 we knew of one star that harbored planets: our own Sun. Since then, planet hunters have expanded this number to a diverse ensemble of over 300 systems orbited by more than 400 individual planets. 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.

My primary research focus is on the detection and characterization of exoplanets. One of my research programs is concerned with studying the relationships between exoplanets and the physical properties of the stars they orbit. The stellar properties provide a vital link between the planets we detect now and the circumstellar environments from which those planets formed long ago. For example, measuring the occurrence rate and properties of planets as a function of stellar mass provides important tests of planet formation theories. I am using the Keck 10m telescope, the world's largest optical facility, to search for planets around stars at either end of the stellar mass scale. On the high-mass end, I am searching for giant planets orbiting "retired" A-type stars, whose progenitors were bright, hot, massive stars like Vega. On the other end of the scale I am searching for planets around low-mass red dwarfs. By comparing the properties of planets around these very different types of stars, I will study how stellar mass affects planet formation.

Gliese 876

Another of my goals in the study of exoplanets is to determine whether worlds like our own exist around other stars. As the precision of Doppler-based planet searches continue to improve, ever smaller exoplanets are being discovered around nearby stars. A very interesting class of planets to emerge are the "super-Earths," which have masses intermediate to the Earth and Neptune (5-10 Earth-masses). I am using high-precision photometry to search for and characterize the transit light curves of these exotic planets. The transit light curve gives a measure of the planet's radius, which together with the planet's mass, gives an average density of the planet. I have designed a new camera customized for high-cadence, sub-millimagnitude photometry, which will be installed on the Palomar 60-inch telescope. The goal of this program is to compare the measured densities of planets to the predictions of theoretical models of planet interiors to determine whether super-Earths are terrestrial, like our own planet, or instead miniature versions of Neptune.

Transit planet

In addition to these projects, I am interested in measuring the spin-orbit alignment of transiting exoplanets using the Rossiter-McLaughlin Effect, in order to test models of planet migration, and to place our Solar System, with its well-aligned planets, in a broader context.

I look forward to working with graduate students at Caltech and I am interested in hearing from you if you have questions about my research, or you have ideas of a project involving exoplanets.

Outside astronomy I enjoy board games of all types, poker, basketball (playing), football (watching) and hanging out with my wife and two sons.

[Image credits: Bob Paz; Trent Schindler/NSF; J. Johnson]