Exploring the Planet-Metallicity Connection with CKS
Happy to announce that the latest paper in California-Kepler Survey has been published in the Astronomical Journal (Petigura et al. 2018). This paper examines the association between a star's chemical composition and the number and type of planets that form. We found that stars rich in heavy elements harbor the greatest diversity of planets.
Stars are mostly made from hydrogen and helium, but they contain trace amounts of other elements like carbon, oxygen, nitrogen, iron, etc—elements that make up the the Earth, life, and us. We can measure the amount of different elements in a star by analysis absorption lines in its spectrum. To astronomers, every element other than hydrogen and helium is a "metal" and a star's overall metal content is known as its metallicity.
Stars and planets are born in clouds of gas and dust like the Orion Nebula, when dense regions collapse under the force of gravity. During this contraction, some of this material flattens out into a protoplanetary disk like a spinning piece of pizza dough. This where planets form. At first, the disk is made of gas and dust, but after about 10 million years, the gas disk is eroded away by high energy radiation. Ten million years is a blink of an eye in the lifetime of a star. Gaseous planets don't have long to form.
The Orion Nebula—a birthplace of stars and planets. Credit: NASA)
We'd like to observe planets forming in realtime. Unfortunately, the vast majority of the known planets orbit stars that are a billion years old or older. However, because the star and the disk form out of the same material, the present day stellar metallicity should trace the composition of its disk, which has long since disappeared. So by studying a star's metallicity and its associated planets, we learn about the processes that link the initial conditions and final outcomes of planet formation.
As part of the California-Kepler Survey, we painstakingly gathered spectra of over 1000 stars hosting planets, measured their metallicities, and looked for associations with the properties of their planets. Some planets, it turns out, are produced with high efficiency regardless of a star's metallicity. Planets smaller than Neptune with orbital periods longer than 10 days are found around stars with wide-ranging metallicities. Larger planets and hotter planets seem to like high metallicity environments. Why?
We don't yet have definitive answer. Metals could assist in the production of large planets by speeding up the formation of their cores. Cores that form early would have more time to accrete gaseous envelopes before the gas in the disk vansishes.
The connection between metals and close-in planets is even more murky. Perhaps metal-rich disks can extend extend closer to their host stars, allowing planets to form with shorter orbits. Or perhaps metal-rich systems are more susceptible dynamical instabilities: violent episodes where planets scatter off one another or merge together. Such episodes may also help to get planets on tight orbits.
While the trends are clear, the interpretation is not. Fortunately, future observations and theoretical work will continue to shed light on the process the connection between stars and their planets.
The number of super-Earths per 100 stars as a function of orbital period and stellar metallicity. Our definition of super-Earth is a planet between 1.0 and 1.7 times the size of the Earth. Super-Earths with orbital periods shorter than 10 days are 3 times more common around stars with super-solar metallicity (red curve) compared to stars with sub-solar metallicity. Figure from Petigura et al. (2018).