Searching for Extrasolar Planets

The most obvious way to search for extrasolar planets is to point a telescope at a nearby star and look for a faint companion. The problem is that even giant planets are a billion times fainter than the star, and their angular separation from the star is typically a small fraction of an arc-second, so the planet is lost in the glare of the overwhelmingly brighter host star.

Wall test velocities.

Wall test velocities.

Astrometry

During most of the 20th century the most promising technique to find an extrasolar planet was precision astrometry. As the planet orbits the star, it kicks the star in a small counter orbit (Newton’s third law: for every action their is an equal and opposite reaction).

planet_reflex_sm

Two bodies with a major difference in mass orbiting around a common barycenter (red cross) with circular orbits.By Rnt20 – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2772661

In principle it is possible to deduce the presence of an extrasolar planet by noting that the precise position of the host star is periodically moving. Peter van de Kamp is especially well known for his pioneering work. Over many decades he took photographic plates of the nearest stars using the 24-inch refracting telescope at the Swarthmore College Sproul Observatory. While this effort did not succeed in finding any confirmed planets, van de Kamp was one of the early proponents of the idea that planetary systems are common.

Peter van de Kamp. Photo courtesy of Swarthmore College.

Peter van de Kamp. Photo courtesy of Swarthmore College.

The development of the “Precision Doppler Velocity” technique began in the 1970s. Over the previous 50 years the measurement uncertainties were 300 m/s or larger, often much larger. It was well known that Jupiter gravitationally tugged the Sun into a small orbit with a velocity of 12 m/s. Since Doppler measurement errors were many hundreds of seconds, this technique was not pursued.

Precision Velocities

In 1973 Griffin & Griffin wrote a seminal paper in which they identified several of the most important sources of measurement uncertainty, and challenged the community to improve velocity precision down to the un-dreamed of level of 10 m/s, sufficient to detect a Jupiter-like planet.

Doppler velocity signal of the Solar System. The orbit of Jupiter is revealed by the 12 year periodicity in the Sun's motion. The 29 year orbital period of Saturn is also evident. With current state-of-the-art Doppler velocity techniques, these are the two Solar System planets that could be detected.

Doppler velocity signal of the Solar System. The orbit of Jupiter is revealed by the 12 year periodicity in the Sun’s motion. The 29 year orbital period of Saturn is also evident. With current state-of-the-art Doppler velocity techniques, these are the two Solar System planets that could be detected.

Bruce Campbell.

Bruce Campbell.

In the late 1970 Bruce Campbell and Gordon Walker conceived of the idea of using a gas absorption cell inserted in the beam of the telescope. The starlight is collected by the primary mirror, and passes through gas absorption cell just prior to entering the spectrometer. The spectrum of the gas vapor in the absorption cell is imprinted on the starlight, and provides a reference spectrum against which to measure the Doppler shift of the star. The reference spectrum is essentially a measuring stick.

Gordon Walker.

Gordon Walker.

Campbell and Walker spent 8 years solving a myriad of problems. Along with their small team, they achieved the critical breakthrough of improving  Doppler velocity measurement precision from 300 m/s to 13 m/s.

The Campbell and Walker gas absorption cell was filled with hydrogen-flouride (HF) vapor, an extremely dangerous gas. Another disadvantage of HF is that it only provides a few reference lines over a limited wavelength range. With so few stellar and reference lines, they were forced to take hour long exposures at the telescope, which limited their survey to about 20 stars.

In September 1986 Geoff Marcy and Paul Butler began exploring the possibility of using a gas other than HF for an absorption cell. The primary goal was to find a gas that provides many more reference lines across a broad region of spectrum where sun-like stars have a significant fraction of their velocity information. This would allow much shorter exposures to reach a precision of 10 m/s, allowing many more stars to be surveyed. A secondary goal of this research was to find a gas that was less toxic than HF.

By ESO - The Radial Velocity Method, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=5168982

The radial velocity method to detect exoplanet is based on the detection of variations in the velocity of the central star, due to the changing direction of the gravitational pull from an (unseen) exoplanet as it orbits the star. When the star moves towards us, its spectrum is blueshifted, while it is redshifted when it moves away from us. By regularly looking at the spectrum of a star – and so, measure its velocity – one can see if it moves periodically due to the influence of a companion. By ESO – The Radial Velocity Method, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=5168982

Butler’s undergraduate degree was in chemistry. He spent the next several months working in the San Francisco State University (SFSU) chemistry labs. Discussions with the chemistry faculty, especially Bill Plachy and Don Eden, were extremely helpful. A number of gases were studied, including, chlorine-dioxide (ClO2) and thioposgene. Hideo Okabe’s classic book, The Photochemistry of Small Molecules, first pointed the way to the possibility of using molecular Iodine (I2).

Along with the SFSU chemistry glass blower, Mylan Healy, Butler built the original Lick Observatory Iodine cell in May 1987. Marcy and Butler first used Iodine cell one month later on the Lick Observatory 3-m telescope. The Iodine cell formed the bulk of Butler’s Physics Masters thesis.

The original Lick Observatory Iodine cell.

The original Lick Observatory Iodine cell.

Like Campbell and Walker before, it took 8 years to solve the myriad of problems, and produce stellar Doppler velocities with a precision of 10 m/s. In November 1994 Steve Vogt upgrading the Hamilton spectometer, more than doubling the wavelength coverage, and dramatically improving the resolution of the spectrometer. By May 1995 the new data was yielding a precision of 3 m/s.

In October 1995 Michel Mayor and Didier Queloz announced the detection of the first extrasolar planet, a hot-jupiter orbiting the nearby star 51 Peg. Within 2 weeks Marcy and Butler were able to confirm this discovery with the Lick Observatory Iodine cell. Over the next 9 months Marcy and Butler discovered the next 5 extrasolar planets from Lick Observatory data.

The first month of observations of 51 Peg from the Lick Observatory Planet Search, October-November 1995.

The first month of observations of 51 Peg from the Lick Observatory Planet Search, October-November 1995.

Over the past 25 years the Iodine absorption cell has become a standard tool for measuring stellar Doppler velocities. Teams at the University of Texas, Penn State, Yale, Harvard, Japan, China, Australia, the European Southern Observatory (VLT/UVES), have all adopted the Iodine technique.