The field of extrasolar planets is new and exciting and one of the surprises was the observation that the planet-hosting stars seemed to have high abundances of heavy elements. To astronomers, all chemical elements beyond hydrogen and helium are "heavy" elements. Both the statistical validity of this early observation and the physical origin of the correlation were hotly disputed.

“Our analysis resulted in a clean mathematical description: the probability of forming a planet increases as the square of the number of heavy-element atoms.”

The two leading theories were: (1) that planets were falling onto the surfaces of stars, polluting the stellar atmospheres with refractory elements—such as iron and silicates—or (2) that stars that formed from metal-rich clouds had the right stuff to make planets more efficiently.

We carried out an extensive and uniform analysis of more than 1,000 stars, a sample size that was 10 times larger than most previous studies. This work decisively quantified the correlation between the formation of extrasolar planets and provided strong support for the second theory: in-falling planets are not responsible for the observed high metallicity of the stars, rather planet formation is enhanced when stars are born in metal-rich molecular clouds.

Credit for the discovery goes to the first scientists to boldly propose a correlation when just a few planets had been discovered (in particular, Guillermo Gonzalez, currently an Assistant Professor of Astronomy at Iowa State University).

Our paper applied a novel analysis with the statistical muscle of more than 1000 stars drawn from the planet search surveys. We did not set out to "look" for a correlation. Rather, we set out to carefully measure the chemical composition of the stars, and, grouping the stars by the strength of their heavy-element abundances, we asked what fraction of stars in each group harbored planets.

Our analysis resulted in a clean mathematical description: the probability of forming a planet increases as the square of the number of heavy-element atoms. This was a key result that went far beyond an anecdotal observation—it hinted at the very formation mechanism for planets as "core accretion" whereby planets build up from small "snowballs" of refractory elements in a protoplanetary disk that encircles the stars. When the planet grows to a few times the mass of our earth, it has enough gravity to rapidly accrete an atmosphere from the gas in the disk and a Jupiter-like planet is born.

When the Milky Way galaxy was first formed, the only chemical elements that existed were hydrogen and helium. As generations of stars manufactured heavier elements in their cores, and then released these elements to the surrounding space when the stars died, new stars formed from these heavier elements.

Because stars in the early galaxy did not have planets or the elements that make up organic life, there were no civilizations in the ancient universe. But when the chemical enrichment of the galaxy reached a critical value, planets were left-over debris in the star formation process.

The cloud that collapsed to form our Sun had enough heavy elements to form our Earth and the other planets in our solar system. This work helps us to understand planet formation and puts some mild constraints on when, in our galactic history, we might expect that planetary systems and life could emerge.

After graduate school, I began working with the renowned planet-hunters, Geoff Marcy of U.C. Berkeley & San Francisco State University and Paul Butler of the Carnegie Institution of Washington’s Department of Terrestrial Magnetism (DTM).

I wanted to better understand the bizarre worlds that we were discovering, and I had unique access to the complete library of stellar spectra for this project. This gave me a unique opportunity to address this question. I used the spectral analysis code that co-author Jeff Valenti developed as a graduate student at UC Berkeley for my thesis work. The code was relatively new, and we would analyze several hundred stars and find trends that revealed a flaw in the code. Jeff would go back to the drawing board and we’d try again.

Altogether, this took several years of trials and failures as we aimed to carry out the best and the most consistent analysis that had ever been done on an enormous sample of stars. We strived for the highest possible precision in our analysis to produce reliable data. Our goal was not to find a correlation; our goal was to produce the best possible data.

This research deepens our understanding of how planets are formed and which stars are most likely to host planetary systems. Knowledge about extrasolar planets plays a role in the selection of some of NASA’s space missions and the search for extrasolar planets is building international scientific collaborations.

For some intellectually curious people, knowledge about other worlds is profoundly important, shaping their philosophy and religion and providing context for the precious value of our world.