Spectral Class Mass (M)
Late M 0.08–0.23
Early M 0.23–0.51
K 0.51–0.79
G 0.79–1.05
Chapter 3
Part I–Minerva: a robotic
telescope array for exoplanet
discovery
Abstract
MINERVA (MINiature Exoplanet Radial Velocity Array) is a robotic observatory for the discovery and characterization of exoplanets. The system is designed for the sensitivity necessary to detect super-Earth type exoplanets around bright, nearby stars. The primary science goal of MINERVA is to be pursued by radial velocity operations; the secondary goal is photometric transit followup of planets. The primary advantage of MINERVA is its modular design and automated exoplanet observations every night, allowing for very high orbital phase coverage and quick turnaround on interesting targets of opportunity. We describe the science goals, design, construction, testing, and verification of MINERVA from its conception as project to its installation as a functioning observatory at Mt. Hopkins in Arizona.
The work and results described in this chapter can be viewed as the first stage of the project, up to installation and verification on Mt. Hopkins. This is not the end of the project, as the spectro- graph installation, verification, and scientific operations are currently ongoing, with the upcoming spectrograph verification being the most critical part.
3.1
Introduction
The two most remarkable findings to come out of the young field of extrasolar planet science are that i) the Galaxy is full of planets; in fact, there are likely more planets than stars, and ii) the frequency of planets increases with decreasing planet size; that is, there are more small, rocky planets than large, gaseous ones (Howard et al., 2011). The latter result is particularly exciting, as recent work has shown that for the case of Earth-sized planets at orbital distances similar to Earth’s, the prevalence is on the order of 10% (Petigura et al., 2013). For smaller M-dwarfs, the most common stars in the Galaxy, the prevalence of habitable-zone Earth-sized planets is similarly calculated to be approximately 15% (Dressing & Charbonneau, 2013). These results are even more impressive in light of the fact that just 25 years ago, there were no known planets outside of our own solar system. Now, there are hundreds of confirmed planets, ranging in size from smaller than Mars to tens of times the mass of Jupiter; orbiting normal sun-like stars, tiny M dwarfs, pulsars (Wolszczan & Frail, 1992), even binary star systems (Doyle et al., 2011). To a large degree, this progress reflects the achievements of the two most successful methods of planet detection. The first is the Doppler method, by which the tiny shift of starlight caused by the gravitational tug of an orbiting planet is measured with respect to a stationary reference source. The second, exemplified by NASAsKepler
mission, is the transit method, by which the dip in intensity of starlight caused by a planet passing in front of its host star is measured.
The two results point to the conclusion that the nearest stars to the Sun should be teeming with planets, particularly low-mass planets of sizes comparable to the Earth and so-called “Super-Earths,” a recently discovered class of planets larger than Earth but smaller than gas giants. However, few of these planetary systems in our solar neighborhood have been discovered. Doppler studies are the most promising way of finding these planets, but the precision required for detecting short-period Earths and habitable-zone Super-Earths is beyond the range of almost all current instruments. Even more important is the tremendous cadence needed to get dense orbital phase coverage, which requires dedicated, repeated observing that is unrealistic within the framework of shared telescope time allocation. This issue is unappreciated, especially as many groups are trying to develop “extreme” precision instruments with the goal of reaching a few cm/s of performance. This level of single-point precision requires long integration times, and without phase coverage does not necessarily increase detection efficiency. Even dedicated exoplanet programs typically are allocated a few dozen telescope nights per year (Sousa et al., 2008; Howard et al., 2010a). The net result is a very poor planetary census of the solar neighborhood.
To fill this scientific need, we are building a dedicated, multi-telescope robotic observatory for detection of extrasolar planets around nearby stars, called the MINiature Exoplanet Radial Velocty Array, or MINERVA . The observatory will pursue exoplanet science every possible night, with the
flexibility of using both precision Doppler spectroscopy or transit photometry. It will consist of four small telescopes which can observe different targets or work together to synthesize a larger effective aperture. It will observe every night, giving an unprecedented level of orbital phase coverage. Finally, MINERVA will be built in a modular fashion, using tested, commercially available technologies, and incorporating the most important lessons from fifteen years worth of ground-based exoplanet instrumentation.
The primary goal of MINERVA will be to discover Earth-like planets in close-in (<50 day) orbits around nearby stars, and super-Earths (M>3 Mearth) in the habitable zones of sun-like stars. This will lead to new, potentially exotic planetary systems, testing planet formation theories, finding targets for the next-generation of space-based missions, and placing the Earth in a broader context among planets in the Solar neighborhood. The target precision of MINERVA is 0.8 m/s, a factor of two better than Keck/HIRES (Howard et al. 2010), which has discovered the most planets by the Doppler method, and comparable to HARPS, the leading European spectrometer (Lovis et al. 2006) The secondary science goal stems from the fast and accurate instrument switching capabilities of the dual Nasmyth ports of our telescopes, allowing us to pursue photometric transit work of our Doppler-detected planets. While the Doppler method measures the mass of a planet, a transit measurement gives the radius. Measuring both the mass and radius can help characterize the interior structures of planets in a mass range not found in our solar system, 3-15 Earth masses.
We performed extensive simulations of the capabilities of MINERVA, considering both instru- mental limitations (spectrograph resolution, dish area, optical efficiency, optical errors, integration time, statistical errors, etc) and fundamental physical limitations (stars spectra, luminosities, dis- tances, spots, rotation rates, etc). We used this performance model along with published Kepler data on planet frequencies to simulate observations over a three-year timescale, including overheads like duty cycle, weather, and other confounding factors. Our simulations indicate that MINERVA will discover 15±4 planets, where 3.2±1.3 will reside within the habitable zone and 1.0 ±0.17 will transit.