KECK OBSERVATORY
The Keck Observatory Planet Search Program discovered most of the first 200 exoplanets, including the first neptune-mass planet and the first super-earth.
As with the Lick Planet Search, the heart of the Keck Planet Search is a spectrometer designed and built by Steve Vogt at the Lick Observatory optical shop. The design for the HIRES spectrometer dates to the late 1980s, long before any extrasolar planets had been discovered. Vogt was aware of the work that Marcy and Butler had been doing on the Hamilton spectrometer at Lick Observatory and commissioned them to build a custom Iodine cell for HIRES.
NASA had purchased a one-sixth share of the two Keck 10-m telescopes. With the discovery of extrasolar planets in 1995 and 1996, and NASAs increasing involvement in the search and characterization of extrasolar planets, the NASA time was heavily weighted toward planet search program. Marcy, Vogt, and Butler started the Keck Planet Search Program on July 10, 1996. The time for the first two night observing run was provided by Ben Zuckerman of UCLA. Shortly thereafter the team began regularly receiving Keck time from NASA and the University of California.
A new planet search program typically requires 2 to 3 years before it begins generating planets. A number of stars need to be surveyed, and about 20 observations covering a full orbit need to be taken.
The first planet to emerge from the Keck Planet Search was the hot jupiter orbiting HD187123, a sun-like star. Kevin Apps a physics and astronomy student at the University of Sussex in the UK advised that this and several other stars be observed. With only 10 planets known at the time, it was already obvious that stars enriched with Iron and other elements heavier than Helium were producing a higher yield of planets than less enriched stars. Kevin studied the literature and found a number of bright enriched stars that were added to the Keck program. Keven shared authorship credit with the Keck team. Within the Keck team, this discovery was known as the “Planet of the Apps”.
HD 187123 was the fourth hot jupiter discovered. Discussion in the community was rampant about the possibility of a transit. Assuming random orbital inclination, the chance of a hot jupiter transiting its star is about 10%.
H209458 was the sixth hot jupiter discovered, and the first to transit its host star. The Keck Doppler velocity data (Henry et al 2000) revealed a planet in 3.5d orbit with a minimum mass of 0.62 jupiter-masses.
Greg Henry at Tennessee State University photometriclly followed up the transit prediction from the Keck Doppler velocity data.
A team headed by David Charbonneau and Tim Brown independently discovered this planet with observations made at the Lowell and Geneva Observatories. The discovery papers by the two groups were published in the same issue of the Astrophysical Journal.
Over the next several years the Keck planet search program repeatedly set the record for the exoplanet with the smallest mass. In 2000 the group published the first two sub-saturn mass planets. HD 16141 has an orbital period of 75.8 days, and a minimum mass of 0.22 Mjup. HD 46375 has a period of 3 days, and a minimum mass of 0.25 Mjup. For comparison, 1 saturn-mass equals 0.30 Mjup.
The Keck planet search program announced the first neptune-mass planet in 2004. GJ 436 b is a neptune-mass planet orbiting a nearby M (red) dwarf star in a 2.64d period. 1 neptune-mass mass equals 17 earth-masses, or 0.054 jupter masses (Mjup)
There are three classes of planets in the Solar System. The terrestrial planets are rocky. The Earth is the most massive example of this class. The ice giants, Neptune and Uranus, are about 15 times more massive than the Earth. Finally the gas giants, Jupiter and Saturn, have masses about 300 and 100 times larger than earth, respectively. In 2005 the Keck planet search announced the first of a new class of planets, super-earths, with masses between 2 and 10 times that of the Earth. The Lick and Keck planet search programs had already discovered two gas giants with orbital periods of 30 and 60 days orbiting the nearby M (red) dwarf star GJ 876. Residuals to the 2-planet fit motivated continuing observations. By 2005 the evidence for a planet in a 1.9d orbit with a minimum mass of 5.9 earth-masses (and a most likely mass of 7.5 earth-masses) had become overwhelming.
The massive outer two planets in this system are in a 2-to-1 orbital resonance. Their gravitational interaction causes their orbital elements to evolve on short timescales, as shown in the middle and bottom panels of the figure.
The original Keck HIRES CCD, installed at first light in 1993 had relatively large pixels (24 microns), a convex surface, excessive charge diffusion (which broadens the point-spread-function), and a subtle nonlinearity in the charge transfer efficiency (CTE). In August 2004 Steve Vogt replaced the original CCD with a mosaic of three CCDs. These CCDs have smaller (15 micron) pixels, a very flat focal plane (improving the the point-spread-function), and more spectral coverage. These CCDs are also free of signal-dependent nonlinearities, and have much smaller charge diffusion.
The Keck planet search program has maintained precision at the level of 3 m/s or better over 20 years. Examples of published stable stars are included below.
