Radial Velocities as a Critical Tool in the Exoplanet
Draft version March 10, 2018 Typeset using LATEX preprint style in AASTeX62
Radial Velocities as a Critical Tool in the Exoplanet Discovery Arsenal: The search for the nearby terrestrial planets with NEID Paul Robertson,1 Suvrath Mahadevan,2, 3 Jason T. Wright,2, 3 Chad F. Bender,4 Cullen Blake,5 David Conran,2, 3 Eric B. Ford,2, 3 Samuel Halverson,5, 6 Fred R. Hearty,2, 3 Shubham Kanodia,2, 3 Kyle F. Kaplan,4 Ming Liang,7 Sarah E. Logsdon,8, 9 Emily Lubar,2, 3 Michael W. McElwain,10 J. P. Ninan,2, 3 Jayadev Rajagopal,7 Lawrence W. Ramsey,2, 3 ´ nsson,2, 3, 14 and Ryan C. Terrien15 Arpita Roy,11 Christian Schwab,12, 13 Gudmundur Stefa 1
Department of Physics and Astronomy, University of California, Irvine, 4129 Frederick Reines Hall, Irvine, CA 92697 USA 2 Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA, USA 3 Center for Exoplanets & Habitable Worlds, The Pennsylvania State University, University Park, PA, USA 4 Steward Observatory, University of Arizona, Tucson, AZ 85721 5 University of Pennsylvania, Physics and Astronomy, Philadelphia, PA, USA 6 NASA Sagan Fellow 7 National Optical Astronomy Observatory 8 NASA Goddard Space Flight Center 9 NASA Postdoctoral Program Fellow 10 Exoplanets and Stellar Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD USA 11 California Institute of Technology, Pasadena, CA, USA 12 Department of Physics and Astronomy, Macquarie University, Sydney, Australia 13 Australian Astronomical Observatory 14 NASA Earth and Space Science Fellow 15 Carleton College, Northfield, MN, USA
Corresponding author: Paul Robertson [email protected]
1 ABSTRACT Over the next decade, efforts to locate and characterize nearby exoplanets will be critically dependent on ultra-precise (σv ∼ 10 cm/s) radial velocity (RV) measurements. As part of the NN-EXPLORE partnership between NASA and the NSF, we are developing the NEID spectrograph, which will achieve 30 cm/s RV measurement precision, with a path to improvement to 10 cm/s. In this white paper, we emphasize that NEID and similar instruments will only achieve their scientific goals with extensive amounts of telescope time. This investment is crucial for the success of programs such as TESS, which rely on ground-based RV follow-up to confirm and characterize candidate exoplanets. 1. INTRODUCTION
Observational exoplanet science will make a significant shift in the next decade from an emphasis on discovering nearby planets to characterizing them. Efforts to explore the nearest planetary systems via spectroscopy and imaging are already underway. The success of these initiatives will be dependent on an order-of-magnitude improvement in sensitivity from radial velocity (RV) spectrographs. Ultra-precise RVs will be required to detect and confirm the most compelling planets for follow-up characterization, including terrestrial planets in the liquid-water habitable zone (HZ). Atmospheric measurements of low-mass planets discovered by TESS (Ricker et al. 2015) will be possible with JWST (Cowan et al. 2015), but RV measurements will be required in order to determine whether a given planet’s atmospheric scale height makes such an observation possible. We are developing the NEID spectrograph (Schwab et al. 2016) for the 3.5 m WIYN Telescope at Kitt Peak National Observatory. Alongside a few other upcoming instruments (see Wright & Robertson 2017, and references therein), NEID is being designed to pave a path to the required orderof-magnitude improvement in RV measurement precision. Our spectrograph design is motivated by a comprehensive, bottom-up instrumental error budget (Halverson et al. 2016), and we expect to go on sky in 2019 with a single-measurement precision of approximately 30 cm/s. With long-term improvements typical of extremely precise Doppler instruments, we hope to identify a realistic path towards 10 cm/s precision. It is important to emphasize that measurement precision alone is insufficient to detect and characterize low-mass exoplanets. Translating exquisite instrumental stability and precision into Doppler sensitivity requires significant investment of telescope time, and the mitigation of astrophysical variability (often referred to as “stellar activity” or “jitter”). In this white paper, we underscore the need for large numbers of well-sampled RV observations in an ultra-precise Doppler survey, and briefly describe how this requirement has shaped our strategy for the NEID guaranteed-time observation (GTO) survey. We outline these considerations in the anticipation that they will be ubiquitous for any scientific programs dependent on state-of-the-art Doppler measurement precision. To aid in these discoveries that are likely essential for future flagship missions to image terrestrial exoplanets, we emphasize the need, both this decade and next: • to dedicate significant amount of time on these modern facilities to obtain a large number of RV observations of bright stars • for telescopes hosting ultra-precise RV instruments to be queue-scheduled to enable these observations
2 • for continued investment in such facilities at 4-8 m telescopes, and continued R&D in increasing RV precision and mitigating the impact of stellar activity 2. RV SENSITIVITY
Due to the effects of non-Gaussian instrumental and algorithmic errors, uneven time sampling, and the still somewhat unpredictable impact of astrophysical variability, it is difficult to precisely translate a Doppler spectrograph’s single-measurement RV precision into a detection sensitivity threshold. However, by assuming RV noise is white to first order, and applying the empirical scaling derived by Plavchan et al. (2015), we may obtain an order-of-magnitude estimate of the number of observations required to achieve sensitivity to RV signals of a given amplitude. Plavchan et al. (2015) surveyed the Doppler amplitudes of exoplanets identified by RV as a function of the number of observations required for those planets’ initial discovery. For the lowest-amplitude planets–which in general required the greatest number of observations–they found that a confident discovery typically involved approximately double the theoretical minimum required under the assumption of purely white noise. In Figure 1, we show this relation as it applies to the 30 cm/s baseline RV precision of NEID, and the resulting Doppler sensitivity for a sample of 45 bright targets. There are two primary consequences of this observation for efforts to discover and characterize terrestrial planets using Doppler instruments with ∼ 10 cm/s single-measurement precision. The first is that even in the limit where our sensitivity approaches the theoretical maximum predicted by the white-noise assumption, RV detection of Earth-mass planets requires many observations–dozens or more, depending on the orbital separation. For exoplanets discovered by transit, the requirement is relaxed by a factor of ∼ 2, but it is important to note that these sensitivity values are only for detection, not detailed characterization. Understanding the details of exoplanet composition and structure requires mass determinations to better than 20% (e.g. Dressing et al. 2015), and constraining orbital parameters such as eccentricity often demands additional observations. Thus, to extract maximal scientific yield from transit surveys such as TESS and PLATO (Rauer et al. 2016), we will likely need more than the bare minimum number of RVs. The second, potentially more daunting implication is that the Plavchan et al. (2015) relation was derived for Doppler instruments for which instrumental error is at least as significant a contributor to the overall RV scatter as astrophysical variability. For the next generation of RV spectrometers, which will approach an order of magnitude better instrumental precision, it is unclear whether this relation will continue to hold. Astrophysical variability, which will dominate the RV scatter of these instruments’ time series, is inherently non-Gaussian, and thus may require even more telescope investment to reliably separate genuine exoplanet signals from false positives and jitter. While modeling astrophysical variability as a signal rather than noise may obviate the need to acquire a greater number of RVs (Jones et al. 2017), the spectral signal-to-noise (i.e. exposure time) requirements for separating activity from Keplerian motion may be greater than would otherwise be necessary for RV precision (Davis et al. 2017). At a minimum, the increased impact of astrophysical variability will place additional constraints on the cadence with which RV observations are collected, as sufficient temporal resolution must be achieved on semi-coherent, decaying stellar signals (e.g. L´opez-Morales et al. 2016). In short, for the next decade’s generation of RV spectrometers to achieve their sensitivity goals and add meaningful contributions to transit surveys, they must receive large telescope allotments that allow for intensive observations over short time spans, but are maintained for multiple seasons,
Number of Queue Nights Per Year 15 30 50
10
RV Amplitude Sensitivity (cm/s)
30
20
3.0
1 MEarth, M* = 0.5 MSun
Em
piri c
The
ore ti
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imu
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015 )
1 MEarth, M* = 0.75 MSun
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1.0
10 1 MEarth, M* = 1.0 MSun
NE009
100
Number of Observations Per Star
1000
Planet Mass Detectable in Habitable Zone of a K2V Star (Earth Masses)
3
Figure 1. RV amplitude sensitivity for a 5σ detection at NEID’s 30 cm/s single-measurement precision, as a function of the number of observations. Dot-dashed lines show amplitudes for M sin i = 1M⊕ planets in the HZs of stars at selected masses. Vertical lines show the number of observations achievable over 5 years for 45 bright stars for selected telescope time allotments. Because signals below 100 cm/s are dominated by stellar activity, the limits shown are approximate, and dependent on activity correction. A large annual time allotment is required for any RV survey to be sensitive to Earth twins around GK stars.
so as to have maximum leverage for distinguishing planets from stellar variability. Additionally, the community must continue its efforts to minimize the effects of astrophysical variability on RV measurements so that terrestrial exoplanets may be recovered with a feasible number of RVs. 3. THE NEID GTO PROGRAM
As shown in Figure 2, NEID’s increase in precision, if achieved, enables RV studies of low-mass planets in a broad new region of parameter space, including the HZs of Sunlike stars. Our GTO program will use NEID to probe deep into this new parameter space to discover “Earth twins:” 1-2 M⊕ planets (RV amplitudes 10-30 cm/s) in the HZs of nearby bright GKM dwarfs. We will target bright (V ≤ 6.5, typical exposure times 3-5 min), nearby stars, selecting from known quiet stars that have already been extensively studied by existing planet search teams (e.g., HARPS, California Planet Search, PARAS) down to precisions of a few m/s. These targets have already been screened for binaries and rapid rotation, and have sufficient observations to accurately model long-period planets and magnetic cycles. Based on the implications of the previous section, the design of our GTO survey must balance acquiring many observations of each target with surveying a large enough sample to expect some to host the planets we wish to discover. Even with a precision of ∼ 30 cm/s, the unambiguous
4
6,500
Venus Earth
6,000
TESS
TESS
(all-sky)
Temperature (K)
5,500
(ecliptic poles)
NEID
NEID
5 nights/year
30 nights/year
5,000
62e
62f 442b
NEID
50 nights/year
4,500 440b
4,000
HARPS
3,500
Mars
(50 cm/s)
438b 296e
186f
296f
Gliese 667Cc
3,000 10
100 Period (days)
1,000
Image Credit: Chester Harman Planets: NASA/JPL/APL/Arrizona/UPR at Arecibo
Figure 2. RV amplitudes for a M sin i = 1M⊕ planet over a range of stellar temperatures and planet orbital periods, and the number of nights required to detect them in a 5-year survey of 45 stars. Colored swaths show the conservative (blue) and optimistic (red+blue) HZ, where water can exist on the surface. For reference, we show a 50 cm/s sensitivity limit, which is approximately equal to the smallest-amplitude planets discovered using HARPS RVs (Pepe et al. 2011; Feng et al. 2017).
detection of exoplanets with signals below 50 cm/s requires hundreds of observations of a star. Estimates of the frequency of Earth twins range from 2-50% (depending on stellar type and HZ bounds; Kopparapu 2013; Foreman-Mackey et al. 2014; Burke et al. 2015), and because any single star may have unfavorable activity properties or planetary orbital inclinations, the sample size must be at least 30 stars, and ideally larger. Our baseline GTO program calls for 30 nights of queuescheduled time per year for five years, so we have chosen a target list that allows us to achieve adequate sampling for 30 stars or more. Our estimate of how many stars may be observed is derived from the exposure time necessary to obtain ∼ 20 cm/s photon noise for a set of real stars from the Eta-Earth survey (Howard et al. 2009), based on our simulation of the NEID throughput and the Doppler information content over our bandpass. It incorporates the number of CCD reads required to avoid nonlinearity, and conservatively
5 assumes 2.5-minute overhead (slew plus acquisition) per observation, based on experience observing with WIYN. Our proposed observing strategy maximizes the precision of NEID by obtaining 100 RVs annually on 45 stars for 5 years. This dense sampling is only possible with NEID’s queue scheduling, and provides enough observations to discover terrestrial-mass planets. Our exposure times will be set to match integer multiples of the (known) p-mode periods of our targets. On the brightest stars, this means we will achieve even lower photon noise, enabling us to probe the ultimate limits of Doppler precision on real stars. 4. CONCLUSION
Conducting high-impact scientific programs with RV in the next decade will require a confluence of state-of-the-art instrumental precision, observational and algorithmic mitigation of astrophysical variability, and a major investment of observing time. For NEID, we have designed a GTO program that has the potential to reveal Earth analogs orbiting nearby stars, but due to the large amount of time required to survey each star, our target list must be kept short. This is certain to be the case for any telescope that awards time competitively to programs in all astrophysical disciplines. Thus, there is a need for both high-precision Doppler spectrographs and extensive observing time on these instruments to meet the demands of programs seeking to discover, confirm, and characterize low-mass exoplanets in the Solar neighborhood. REFERENCES Burke, C. J., Christiansen, J. L., Mullally, F., et al. 2015, ApJ, 809, 8 Cowan, N. B., Greene, T., Angerhausen, D., et al. 2015, PASP, 127, 311 Davis, A. B., Cisewski, J., Dumusque, X., Fischer, D. A., & Ford, E. B. 2017, ApJ, 846, 59 Dressing, C. D., Charbonneau, D., Dumusque, X., et al. 2015, ApJ, 800, 135 Feng, F., Tuomi, M., Jones, H. R. A., et al. 2017, AJ, 154, 135 Foreman-Mackey, D., Hogg, D. W., & Morton, T. D. 2014, ApJ, 795, 64 Halverson, S., Terrien, R., Mahadevan, S., et al. 2016, Proc. SPIE, 9908, 99086P Howard, A. W., Johnson, J. A., Marcy, G. W., et al. 2009, ApJ, 696, 75 Jones, D. E., Stenning, D. C., Ford, E. B., et al. 2017, arXiv:1711.01318
Kopparapu, R. K. 2013, ApJL, 767, L8 L´opez-Morales, M., Haywood, R. D., Coughlin, J. L., et al. 2016, AJ, 152, 204 Pepe, F., Lovis, C., S´egransan, D., et al. 2011, A&A, 534, A58 Plavchan, P., Latham, D., Gaudi, S., et al. 2015, arXiv:1503.01770 Rauer, H., Aerts, C., Cabrera, J., & PLATO Team 2016, Astronomische Nachrichten, 337, 961 Ricker, G. R., Winn, J. N., Vanderspek, R., et al. 2015, Journal of Astronomical Telescopes, Instruments, and Systems, 1, 014003 Schwab, C., Rakich, A., Gong, Q., et al. 2016, Proc. SPIE, 9908, 99087H Wright, J. T., & Robertson, P. 2017, Research Notes of the American Astronomical Society, 1, 51
Radial Velocities as a Critical Tool in the Exoplanet Discovery Arsenal: The search for the nearby terrestrial planets with NEID Paul Robertson,1 Suvrath Mahadevan,2, 3 Jason T. Wright,2, 3 Chad F. Bender,4 Cullen Blake,5 David Conran,2, 3 Eric B. Ford,2, 3 Samuel Halverson,5, 6 Fred R. Hearty,2, 3 Shubham Kanodia,2, 3 Kyle F. Kaplan,4 Ming Liang,7 Sarah E. Logsdon,8, 9 Emily Lubar,2, 3 Michael W. McElwain,10 J. P. Ninan,2, 3 Jayadev Rajagopal,7 Lawrence W. Ramsey,2, 3 ´ nsson,2, 3, 14 and Ryan C. Terrien15 Arpita Roy,11 Christian Schwab,12, 13 Gudmundur Stefa 1
Department of Physics and Astronomy, University of California, Irvine, 4129 Frederick Reines Hall, Irvine, CA 92697 USA 2 Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA, USA 3 Center for Exoplanets & Habitable Worlds, The Pennsylvania State University, University Park, PA, USA 4 Steward Observatory, University of Arizona, Tucson, AZ 85721 5 University of Pennsylvania, Physics and Astronomy, Philadelphia, PA, USA 6 NASA Sagan Fellow 7 National Optical Astronomy Observatory 8 NASA Goddard Space Flight Center 9 NASA Postdoctoral Program Fellow 10 Exoplanets and Stellar Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD USA 11 California Institute of Technology, Pasadena, CA, USA 12 Department of Physics and Astronomy, Macquarie University, Sydney, Australia 13 Australian Astronomical Observatory 14 NASA Earth and Space Science Fellow 15 Carleton College, Northfield, MN, USA
Corresponding author: Paul Robertson [email protected]
1 ABSTRACT Over the next decade, efforts to locate and characterize nearby exoplanets will be critically dependent on ultra-precise (σv ∼ 10 cm/s) radial velocity (RV) measurements. As part of the NN-EXPLORE partnership between NASA and the NSF, we are developing the NEID spectrograph, which will achieve 30 cm/s RV measurement precision, with a path to improvement to 10 cm/s. In this white paper, we emphasize that NEID and similar instruments will only achieve their scientific goals with extensive amounts of telescope time. This investment is crucial for the success of programs such as TESS, which rely on ground-based RV follow-up to confirm and characterize candidate exoplanets. 1. INTRODUCTION
Observational exoplanet science will make a significant shift in the next decade from an emphasis on discovering nearby planets to characterizing them. Efforts to explore the nearest planetary systems via spectroscopy and imaging are already underway. The success of these initiatives will be dependent on an order-of-magnitude improvement in sensitivity from radial velocity (RV) spectrographs. Ultra-precise RVs will be required to detect and confirm the most compelling planets for follow-up characterization, including terrestrial planets in the liquid-water habitable zone (HZ). Atmospheric measurements of low-mass planets discovered by TESS (Ricker et al. 2015) will be possible with JWST (Cowan et al. 2015), but RV measurements will be required in order to determine whether a given planet’s atmospheric scale height makes such an observation possible. We are developing the NEID spectrograph (Schwab et al. 2016) for the 3.5 m WIYN Telescope at Kitt Peak National Observatory. Alongside a few other upcoming instruments (see Wright & Robertson 2017, and references therein), NEID is being designed to pave a path to the required orderof-magnitude improvement in RV measurement precision. Our spectrograph design is motivated by a comprehensive, bottom-up instrumental error budget (Halverson et al. 2016), and we expect to go on sky in 2019 with a single-measurement precision of approximately 30 cm/s. With long-term improvements typical of extremely precise Doppler instruments, we hope to identify a realistic path towards 10 cm/s precision. It is important to emphasize that measurement precision alone is insufficient to detect and characterize low-mass exoplanets. Translating exquisite instrumental stability and precision into Doppler sensitivity requires significant investment of telescope time, and the mitigation of astrophysical variability (often referred to as “stellar activity” or “jitter”). In this white paper, we underscore the need for large numbers of well-sampled RV observations in an ultra-precise Doppler survey, and briefly describe how this requirement has shaped our strategy for the NEID guaranteed-time observation (GTO) survey. We outline these considerations in the anticipation that they will be ubiquitous for any scientific programs dependent on state-of-the-art Doppler measurement precision. To aid in these discoveries that are likely essential for future flagship missions to image terrestrial exoplanets, we emphasize the need, both this decade and next: • to dedicate significant amount of time on these modern facilities to obtain a large number of RV observations of bright stars • for telescopes hosting ultra-precise RV instruments to be queue-scheduled to enable these observations
2 • for continued investment in such facilities at 4-8 m telescopes, and continued R&D in increasing RV precision and mitigating the impact of stellar activity 2. RV SENSITIVITY
Due to the effects of non-Gaussian instrumental and algorithmic errors, uneven time sampling, and the still somewhat unpredictable impact of astrophysical variability, it is difficult to precisely translate a Doppler spectrograph’s single-measurement RV precision into a detection sensitivity threshold. However, by assuming RV noise is white to first order, and applying the empirical scaling derived by Plavchan et al. (2015), we may obtain an order-of-magnitude estimate of the number of observations required to achieve sensitivity to RV signals of a given amplitude. Plavchan et al. (2015) surveyed the Doppler amplitudes of exoplanets identified by RV as a function of the number of observations required for those planets’ initial discovery. For the lowest-amplitude planets–which in general required the greatest number of observations–they found that a confident discovery typically involved approximately double the theoretical minimum required under the assumption of purely white noise. In Figure 1, we show this relation as it applies to the 30 cm/s baseline RV precision of NEID, and the resulting Doppler sensitivity for a sample of 45 bright targets. There are two primary consequences of this observation for efforts to discover and characterize terrestrial planets using Doppler instruments with ∼ 10 cm/s single-measurement precision. The first is that even in the limit where our sensitivity approaches the theoretical maximum predicted by the white-noise assumption, RV detection of Earth-mass planets requires many observations–dozens or more, depending on the orbital separation. For exoplanets discovered by transit, the requirement is relaxed by a factor of ∼ 2, but it is important to note that these sensitivity values are only for detection, not detailed characterization. Understanding the details of exoplanet composition and structure requires mass determinations to better than 20% (e.g. Dressing et al. 2015), and constraining orbital parameters such as eccentricity often demands additional observations. Thus, to extract maximal scientific yield from transit surveys such as TESS and PLATO (Rauer et al. 2016), we will likely need more than the bare minimum number of RVs. The second, potentially more daunting implication is that the Plavchan et al. (2015) relation was derived for Doppler instruments for which instrumental error is at least as significant a contributor to the overall RV scatter as astrophysical variability. For the next generation of RV spectrometers, which will approach an order of magnitude better instrumental precision, it is unclear whether this relation will continue to hold. Astrophysical variability, which will dominate the RV scatter of these instruments’ time series, is inherently non-Gaussian, and thus may require even more telescope investment to reliably separate genuine exoplanet signals from false positives and jitter. While modeling astrophysical variability as a signal rather than noise may obviate the need to acquire a greater number of RVs (Jones et al. 2017), the spectral signal-to-noise (i.e. exposure time) requirements for separating activity from Keplerian motion may be greater than would otherwise be necessary for RV precision (Davis et al. 2017). At a minimum, the increased impact of astrophysical variability will place additional constraints on the cadence with which RV observations are collected, as sufficient temporal resolution must be achieved on semi-coherent, decaying stellar signals (e.g. L´opez-Morales et al. 2016). In short, for the next decade’s generation of RV spectrometers to achieve their sensitivity goals and add meaningful contributions to transit surveys, they must receive large telescope allotments that allow for intensive observations over short time spans, but are maintained for multiple seasons,
Number of Queue Nights Per Year 15 30 50
10
RV Amplitude Sensitivity (cm/s)
30
20
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1 MEarth, M* = 0.5 MSun
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Planet Mass Detectable in Habitable Zone of a K2V Star (Earth Masses)
3
Figure 1. RV amplitude sensitivity for a 5σ detection at NEID’s 30 cm/s single-measurement precision, as a function of the number of observations. Dot-dashed lines show amplitudes for M sin i = 1M⊕ planets in the HZs of stars at selected masses. Vertical lines show the number of observations achievable over 5 years for 45 bright stars for selected telescope time allotments. Because signals below 100 cm/s are dominated by stellar activity, the limits shown are approximate, and dependent on activity correction. A large annual time allotment is required for any RV survey to be sensitive to Earth twins around GK stars.
so as to have maximum leverage for distinguishing planets from stellar variability. Additionally, the community must continue its efforts to minimize the effects of astrophysical variability on RV measurements so that terrestrial exoplanets may be recovered with a feasible number of RVs. 3. THE NEID GTO PROGRAM
As shown in Figure 2, NEID’s increase in precision, if achieved, enables RV studies of low-mass planets in a broad new region of parameter space, including the HZs of Sunlike stars. Our GTO program will use NEID to probe deep into this new parameter space to discover “Earth twins:” 1-2 M⊕ planets (RV amplitudes 10-30 cm/s) in the HZs of nearby bright GKM dwarfs. We will target bright (V ≤ 6.5, typical exposure times 3-5 min), nearby stars, selecting from known quiet stars that have already been extensively studied by existing planet search teams (e.g., HARPS, California Planet Search, PARAS) down to precisions of a few m/s. These targets have already been screened for binaries and rapid rotation, and have sufficient observations to accurately model long-period planets and magnetic cycles. Based on the implications of the previous section, the design of our GTO survey must balance acquiring many observations of each target with surveying a large enough sample to expect some to host the planets we wish to discover. Even with a precision of ∼ 30 cm/s, the unambiguous
4
6,500
Venus Earth
6,000
TESS
TESS
(all-sky)
Temperature (K)
5,500
(ecliptic poles)
NEID
NEID
5 nights/year
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62f 442b
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4,500 440b
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HARPS
3,500
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(50 cm/s)
438b 296e
186f
296f
Gliese 667Cc
3,000 10
100 Period (days)
1,000
Image Credit: Chester Harman Planets: NASA/JPL/APL/Arrizona/UPR at Arecibo
Figure 2. RV amplitudes for a M sin i = 1M⊕ planet over a range of stellar temperatures and planet orbital periods, and the number of nights required to detect them in a 5-year survey of 45 stars. Colored swaths show the conservative (blue) and optimistic (red+blue) HZ, where water can exist on the surface. For reference, we show a 50 cm/s sensitivity limit, which is approximately equal to the smallest-amplitude planets discovered using HARPS RVs (Pepe et al. 2011; Feng et al. 2017).
detection of exoplanets with signals below 50 cm/s requires hundreds of observations of a star. Estimates of the frequency of Earth twins range from 2-50% (depending on stellar type and HZ bounds; Kopparapu 2013; Foreman-Mackey et al. 2014; Burke et al. 2015), and because any single star may have unfavorable activity properties or planetary orbital inclinations, the sample size must be at least 30 stars, and ideally larger. Our baseline GTO program calls for 30 nights of queuescheduled time per year for five years, so we have chosen a target list that allows us to achieve adequate sampling for 30 stars or more. Our estimate of how many stars may be observed is derived from the exposure time necessary to obtain ∼ 20 cm/s photon noise for a set of real stars from the Eta-Earth survey (Howard et al. 2009), based on our simulation of the NEID throughput and the Doppler information content over our bandpass. It incorporates the number of CCD reads required to avoid nonlinearity, and conservatively
5 assumes 2.5-minute overhead (slew plus acquisition) per observation, based on experience observing with WIYN. Our proposed observing strategy maximizes the precision of NEID by obtaining 100 RVs annually on 45 stars for 5 years. This dense sampling is only possible with NEID’s queue scheduling, and provides enough observations to discover terrestrial-mass planets. Our exposure times will be set to match integer multiples of the (known) p-mode periods of our targets. On the brightest stars, this means we will achieve even lower photon noise, enabling us to probe the ultimate limits of Doppler precision on real stars. 4. CONCLUSION
Conducting high-impact scientific programs with RV in the next decade will require a confluence of state-of-the-art instrumental precision, observational and algorithmic mitigation of astrophysical variability, and a major investment of observing time. For NEID, we have designed a GTO program that has the potential to reveal Earth analogs orbiting nearby stars, but due to the large amount of time required to survey each star, our target list must be kept short. This is certain to be the case for any telescope that awards time competitively to programs in all astrophysical disciplines. Thus, there is a need for both high-precision Doppler spectrographs and extensive observing time on these instruments to meet the demands of programs seeking to discover, confirm, and characterize low-mass exoplanets in the Solar neighborhood. REFERENCES Burke, C. J., Christiansen, J. L., Mullally, F., et al. 2015, ApJ, 809, 8 Cowan, N. B., Greene, T., Angerhausen, D., et al. 2015, PASP, 127, 311 Davis, A. B., Cisewski, J., Dumusque, X., Fischer, D. A., & Ford, E. B. 2017, ApJ, 846, 59 Dressing, C. D., Charbonneau, D., Dumusque, X., et al. 2015, ApJ, 800, 135 Feng, F., Tuomi, M., Jones, H. R. A., et al. 2017, AJ, 154, 135 Foreman-Mackey, D., Hogg, D. W., & Morton, T. D. 2014, ApJ, 795, 64 Halverson, S., Terrien, R., Mahadevan, S., et al. 2016, Proc. SPIE, 9908, 99086P Howard, A. W., Johnson, J. A., Marcy, G. W., et al. 2009, ApJ, 696, 75 Jones, D. E., Stenning, D. C., Ford, E. B., et al. 2017, arXiv:1711.01318
Kopparapu, R. K. 2013, ApJL, 767, L8 L´opez-Morales, M., Haywood, R. D., Coughlin, J. L., et al. 2016, AJ, 152, 204 Pepe, F., Lovis, C., S´egransan, D., et al. 2011, A&A, 534, A58 Plavchan, P., Latham, D., Gaudi, S., et al. 2015, arXiv:1503.01770 Rauer, H., Aerts, C., Cabrera, J., & PLATO Team 2016, Astronomische Nachrichten, 337, 961 Ricker, G. R., Winn, J. N., Vanderspek, R., et al. 2015, Journal of Astronomical Telescopes, Instruments, and Systems, 1, 014003 Schwab, C., Rakich, A., Gong, Q., et al. 2016, Proc. SPIE, 9908, 99087H Wright, J. T., & Robertson, P. 2017, Research Notes of the American Astronomical Society, 1, 51
