Anglo-Australian Telescope

The Anglo-Australian Planet Search program has discovered more than 40 planets orbiting nearby stars.

The Australian Astronomical Observatory, a division of the Department of Industry, Innovation and Science, operates the Anglo-Australian and UK Schmidt telescopes on behalf of the astronomical community of Australia. Image from AAO.

The Australian Astronomical Observatory, a division of the Department of Industry, Innovation and Science, operates the Anglo-Australian and UK Schmidt telescopes on behalf of the astronomical community of Australia. Image from AAO website.

In August 1997 Paul Butler accepted a position as a staff astronomer at the Anglo-Australian Observatory and moved to Sydney Australia. The primary goal of taking the AAO position was to initiate a southern hemisphere planet search to complement the programs at the Lick and Keck Observatories. Butler recruited Chris Tinney (then an AAO staff astronomer, currently a professor at UNSW) and Hugh Jones (then a professor at Liverpool John Moores University, currently a professor at the University of Hertfordshire, near London).

Other important AAPS team members include Brad Carter (professor at the University of Southern Queensland), Jeremy Bailey (then at the AAO, currently a professor at UNSW), Simon O’Toole (AAO), and Rob Wittenmyer (then a professor at UNSW, currently a professor at the University of Southern Queensland).

The Anglo-Australian Planet Search (AAPS) program makes use of the University London Echelle Spectrometer (UCLES), which had been installed at the Anglo-Australian Telescope (AAT) in 1990. Butler provided the Iodine absorption cell. In January 1998 the CCD detector for UCLES was upgraded, and the AAPS program began taking data.

The first year of the AAPS program coincided with one of the wettest years on record at Siding Spring Observatory. Of the 28 nights assigned to the AAPS program, the telescope was only open on 8 nights, many of these nights were afflicted with cloud and rain.

As is typical in precision Doppler surveys, the planets began to emerge from this program after three years. The first planets published from this program were a hot jupiter orbiting HD 179949 (Tinney et al. 2001), an eccentric giant planet orbiting HD 160691, and giant planet in a circular planet similar to the Earth’s orbiting HD 27442 (Butler et al. 2001). The planet around HD 27442 was one of the first planets found in a circular orbit beyond the tidal circularization zone around the host star.

Nature prefers to build giant planets around sun-like stars that are enriched in atoms heavier than hydrogen and helium. The 2003 AAPS paper “Four New Planets Orbiting Metal-enriched Stars” (Tinney et al.) was an early data rich proponent of focusing on metal-rich stars.

The biggest looming questions in exoplanet science are what fraction of planetary systems resemble the Solar System, and which nearby stars have potentially habitable planets. Finding Solar System analogs is difficult. With current technology, Jupiter and Saturn-analogs are the only detectable signposts of Solar System architecture, giant planets in circular orbits beyond 3 AU. The giant planet orbiting HD 70642 (Carter et al. 2003) was one of the first examples of this sort of planet.

The first multiple planet system was only discovered in 1999. Over the next several years multiple planet systems began to slowly emerge. Two early multiple planet systems were discovered by the AAPS (McCarthy et al 2004). The discovery data for the double planet system around HD 160691 is shown below.

A double Keplerian fit to HD 160691. The inner Keplerian has a period of 645 days, an eccentricity of 0.20, and a semiamplitude of 38 m s-1, yielding a minimum mass of 1.68MJ and a semimajor axis of 1.50 AU. The outer Keplerian has a period of 8.2 yr, an eccentricity of 0.57, and a semiamplitude of 51 m s-1, yielding a minimum mass of 3.1MJ and a semimajor axis of 4.16 AU. The rms to this model is 4.66 m s-1 (1.69 σ). (McCarthy et al. 2004)

A double Keplerian fit to HD 160691. The inner Keplerian has a period of 645 days, an eccentricity of 0.20, and a semiamplitude of 38 m s-1, yielding a minimum mass of 1.68MJ and a semimajor axis of 1.50 AU. The outer Keplerian has a period of 8.2 yr, an eccentricity of 0.57, and a semiamplitude of 51 m s-1, yielding a minimum mass of 3.1MJ and a semimajor axis of 4.16 AU. The rms to this model is 4.66 m s-1 (1.69 σ). (McCarthy et al. 2004)

In January 2007 the AAPS began the first of two long observing campaigns. About 20 bright stars were observed multiple times each night over 48 nights. This data set, combined with data from the Keck/HIRES program, was crucial in discovering the multiple planet system orbiting 61 Vir, including the 5 earth-mass “super-earth” in a 4.2 day orbit(Vogt et al. 2010). This was one of the first “super-earths”, a new class of planet not found in the Solar Sytem, but now known to be extremely common. This was also the harbinger of a new class of planetary systems, packed with planets in the range of a few earth-masses to a few neptune-masses.

AAT velocities of HD 115617 taken in January/February 2007. The 5 earth-mass planet in a 4.2 day orbit, and the neptune-mass planet in a 38 day orbit are apparent. While systems like this are now known to be common, in 2010 this was a real oddball.

AAT velocities of HD 115617 taken in January/February 2007. The 5 earth-mass planet in a 4.2 day orbit, and the neptune-mass planet in a 38 day orbit are apparent. While systems like this are now known to be common, in 2010 this was a real oddball.

The faint red M dwarf stars are now known to commonly host planetary systems with super-earth and neptune-mass planets. Giant planets orbiting M dwarfs remains relatively rare. One of the first such planets was found around GJ832 (Bailey et al. 2009). This planet remains interesting as it is a jupiter-analog in a 9.4 year period. This star has since been found to also host a 5 earth-mass planet in a 35 day orbit.

Doppler velocities for GJ 832 spanning 9.6 yr. The upper panel shows the measured velocities with a best-fit Keplerian over-plotted as a dashed line. The residuals to this fit are plotted in the lower panel.

Doppler velocities for GJ 832 spanning 9.6 yr. The upper panel shows the measured velocities with a best-fit Keplerian over-plotted as a dashed line. The residuals to this fit are plotted in the lower panel.

The AAPS has maintained a precision of 3 m/s over the nearly twenty years of the program. Examples of stable stars include G and early K dwarfs (McCarthy et al. 2004.), and late K and M dwarfs (Bailey et al. 2009).

AAT Doppler velocities of chromospherically quiet G and K dwarfs and subgiants. These observations span the first six years of the AAT Planet Search Project. (McCarthy et al. 2004)

AAT Doppler velocities of chromospherically quiet G and K dwarfs and subgiants. These observations span the first six years of the AAT Planet Search Project. (McCarthy et al. 2004)

Stable late K and early M dwarfs observed with the AAT. The velocity precision (~5 m/s) is worse than the G dwarfs because these stars are significantly fainter.

Stable late K and early M dwarfs observed with the AAT. The velocity precision (~5 m/s) is worse than the G dwarfs because these stars are significantly fainter.