Imaging and Spectra of Exoplanets Orbiting our Nearest Sun-Like Star Neighbors with a Starshade in the 2020s

Transcription

1 White Paper on Imaging and Spectra of Exoplanets Orbiting our Nearest Sun-Like Star Neighbors with a Starshade in the 2020s A Submission to the National Academy of Sciences Authors: Sara Seager, Massachusetts Institute of Technology, seager@mit.edu, Jeremy Kasdin, Princeton University, jkasdin@princeton.edu, Co-authors: Andrew Gray, JPL, Jeff Booth, JPL, Matt Greenhouse, GSFC, Doug Lisman JPL, Stuart Shaklan, JPL, Maggie Turnbull, Global Sci., Melissa Vess, GSFC, Steve Warwick, NGC, David Webb, JPL, Jon Arenberg, NGC, Shawn Domagal-Goldman, GSFC, Sergi Hildebrandt, JPL, Renyu Hu, JPL, Michael Hughes, JPL, Alina Kiessling, JPL, Nikole Lewis, StScI, Eric Mamajek, JPL, Jason Rhodes, JPL, Leslie Rogers, University of Chicago, Aki Roberge, GSFC, Andrew Romero-Wolf, JPL, Dmitry Savransky, Cornell University, Chris Stark, StScI, John Ziemer, JPL. March 9, 2018 Jet Propulsion Laboratory California Institute of Technology Pasadena, CA 91109

2 SUMMARY A starshade working with a small telescope (1 to 2.4 m-class size) offers unprecedented spacebased direct imaging of exoplanets. Focusing on our nearest one to two dozen sun-like star neighbors, such a starshade (20-40m in diameter) can enable spectra for entire planetary systems including giant planets, dust disks, and Earth analogs, providing a holistic understanding of our nearest planetary systems. Plans are in place for the starshade mission to be ready for launch in the mid-2020s, thus opening a new era of exoplanet discovery and characterization. 1. WHY DIRECT IMAGING FROM SPACE? Space based direct imaging is the only way to discover and study atmospheres of Earth-sized planets around the nearest sun-like stars (Figure 1). The past two decades have seen enormous advances in exoplanet science, largely from discoveries using radial velocity and transit techniques. The explosion of planets discovered by Kepler has taught us about the diversity of planetary systems, most very different from our own solar system. New planet types have been discovered that do not fit into existing formation models. The first spectra of young, cool Jupiter-size planets from GPI, SPHERE, and SCExAO/CHARIS have provided new and important data for these modeling efforts. The first Earth- and super-earth size planets have been found orbiting cool M dwarf stars [1,2,3,4] capturing the imagination of scientists and the public alike. All of these techniques, telescopes, and instruments have dramatically expanded what we know about planet occurrence rates, orbital distributions, and planetary types. Figure 1. Exoplanets from different discovery techniques. Atmosphere observations (highlighted by circles) can be only observed for transiting planets (small semi- major axis) or directly imaged planets (large semi- major axis). Space- based starshade direct imaging (box) is needed to study Earth- sized planet atmospheres around sun- like stars, inaccessible by transit transmission spectra (extreme low probability (1/200) for 1 AU orbiting planets to transit sun- like stars) and also inaccessible by ground- based imaging (see contrast vs. separation figure in Kasdin et al. white paper). Transit spectroscopy and direct imaging from the ground have given us a taste of the complexity of exoplanet atmospheres. Upcoming missions such as TESS and JWST will build on this base, exploring a larger portion of the sky and enabling higher fidelity transit spectroscopy for exoplanets transiting M dwarf stars. Yet there is still a large portion of parameter space unexplored, and mostly unreachable, by these techniques (Figure 1). Discovering planets from Jupiter at 5 AU down to Earth size in the habitable zones of sun-like stars, and characterizing their atmospheres, requires direct imaging. Current ground-based instruments are limited to large, distant, self-luminous planets; imaging 1

3 and characterizing true Earth analogs requires going to space. discovery in exoplanet science. This is the next frontier of 2. WHY A STARSHADE? The starshade is a powerful technology for space-based direct imaging of exoplanets, one that simplifies demands on the telescope compared to other starlight suppression techniques. A starshade is a large, precisely shaped screen, tens of meters in diameter, flying in formation with a distant telescope situated tens of thousands of km away (Figure 2 and [5]). The starshade blocks unwanted starlight, creating a shadow where the telescope lies, allowing only planet light to enter the telescope. Built to tolerances of hundreds of microns for petal shape, tens of mm for petal positioning, and formation flying to 150 km along the line of sight and meters laterally, the starshade can reach inner working angles (IWA) of 80 to 100 mas, and planet-to-star contrasts of (though for the nearest sun-like stars an IWA of only 150 mas is needed). The current state-of-the art in direct imaging and near future capabilities are shown in other direct imaging white papers (Kasdin et al.; Mennesson et al.; Bailey et al.). For context a starshade with WFIRST (or other similar sized telescope) will be capable of detecting Earths at a planet-to-star flux ratio of (Note that the starshade mission is designed to reach a planet detection sensitivity of 4 x after allowing for integration times, to enable planet discovery away from the optimal planet phase.) A starshade enables exquisite detection limits because the contrast and IWA largely decouple from telescope aperture size. Plus, the outer working angle is limited only by the size of the detector. Use of a starshade also relaxes requirements on the optical quality of the telescope; since no starlight enters the telescope, no complex wavefront control is necessary. Other advantages of a starshade are its high throughput and broad wavelength bandpass (400 nm to 1000 nm). These two key features enable high sensitivity spectroscopy for characterization at planet-star contrasts of 10-10, unique amongst starlight suppression techniques. Because of the high throughput, a small telescope is sufficient for obtaining spectra of exoearths [5,6]. The number of nearby target stars available is well matched to the number of starshade retargeting maneuvers, mitigating the main starshade challenge of repositioning for target stars. 3. WHY NOW? Figure 2. Artist s conception of the Starshade, from the cover of [5]. Why now for science? Within a decade, a starshade mission could discover the first Earth-sized exoplanets in Earth-like orbits about sun-like stars and obtain spectra of their atmospheres. We envision a deep dive focused on our nearest sun-like neighbors, scouring the systems for all of their contents. The discovery of another Earth would reverberate far beyond astronomy. Why now for technology? Conceived in the 1960s, and revisited each decade since, starshade technology now heavily builds upon deep heritage from large space-based radio deployables. Because the burden of starlight suppression is on the starshade, no new technologies for the space telescope are needed. A mission using a starshade of m in diameter is executable by 2

4 Figure 3. Simulated spectra of exoplanets as observed with a Starshade mission, showing different types of planets can be identified from Starshade observations. The spectra have been convolved to R= 70 spectral resolution and re- binned onto a wavelength grid with 10 nm bins; noise is not included. Left: Spectra of small planets (spectral models from VPL and R. Hu). Right: Geometric albedo spectra of real and modeled giant planets [8,9]. From [5]. NASA from a technical perspective in the 2020s, while larger starshades need a longer development time. A directed effort to mature five different starshade technologies to TRL 5 by 2022 was created by NASA s Astrophysics Division in 03/2016 (called Starshade to TRL 5 or S5), though shorter timescales are possible with more funding. Progress on these key technologies is described in an accompanying white paper by Ziemer et al. Major risks will be retired by 2019 with no additional new technologies needed beyond those addressed by S5. Why now programmatically? The stakes for space-based direct imaging of exoplanets in this next Decadal Survey cycle could not be higher. NASA s serial development approach to strategic class missions means that foundational missions with technology as close to heritage as possible significantly reduce development risk. The mission concepts targeting a significantly greater number of target stars than a first starshade mission is capable of, operating in the 2030s and beyond (currently under study by NASA for the 2020 survey), should build upon the technical and scientific heritage obtained by development and operation of the smaller starshade mission in the 2020s. The first opportunity is the Starshade Rendezvous mission with WFIRST. 4. SCIENCE WITH A STARSHADE IN THE 2020S In 2013 NASA commissioned the Exo-S study team to examine a Probe-Class mission using a starshade with a small telescope [5]. That led to an extended study and a new probe study, started in 2017, to update earlier work and examine the scientific yield of a starshade in combination with WFIRST, dubbed the Starshade Rendezvous Mission. An exoplanet imaging and spectroscopy mission with a starshade has four science goals. The first goal is to discover new planets from Earth size to giant planets. Within this goal is the possibility of discovering Earthsize exoplanets in the habitable zones (HZ) of over a dozen Sun-like stars arguably one of the most exciting pursuits in exoplanet research. The second science goal is to measure spectra of a subset of newly discovered planets. The Exo- S spectral range is from 400 1,000 nm, with a spectral resolution of up to R=70, which enables detection of key spectral features (Figure 3). Of particular interest is the detection of water vapor and oxygen on small exoplanets. 3

5 The third science goal is to characterize known giant planets, by obtaining spectra and measuring or constraining planet masses. Known giant planets are detectable by virtue of extrapolated position in the mid 2020s timeframe. Molecular composition and the presence/absence of clouds or hazes will yield information on the diversity of giant planet atmospheres, enhancing our knowledge of planetary systems and formation mechanisms. The fourth science goal is to characterize the dynamic evolution of planetary systems, with a specific focus on studying circumstellar dust in the context of known planets. Observations will shed light on the dust-generating parent bodies (asteroids and comets), and the system s dynamical history, as well as possibly pointing to unseen planets below the mission s direct detection thresholds. An assessment of dust levels in the habitable zones of nearby stars (exozodi) is a significant unknown affecting mission planning for future flagship mission concepts. The science yield, in terms of how many planets are discovered and at what spectral resolution small planet atmospheres can be characterized, depends on the telescope aperture, the backend instrument, and the observing strategy, i.e., how a finite number of starshade retargets are allocated. Discovery integration times are on the order of hours, spectroscopy times are on the order of one week, and retarget times are on the order of one week. Proofof-concept design reference missions show that enough fuel is available for retargeting to meet the science goals presented in [5,6]. See Fig. 4. Figure 4. One example of a conservative estimated starshade planet discovery yield f in the first two years of operations considering 9% of telescope time for two years on a 2.4 m telescope. From [5]. The starshade science case for a 26 m starshade and a 1.1 m telescope was recently updated for a three-year mission [6]. Planet yield was simulated with the EXOSIMS code [9] using the assumed exoplanet population distributions and characteristics adopted by the ExSDET (SAG13). The number of predicted detections of exoplanets in a two-year mission is over 45 new planets with 8 of them rocky planets (~3 in the HZ). Two thirds of the starshade fuel is left over for a third year for characterization and further revisits. 5. TWO MISSION CONCEPTS Two concepts for a starshade mission in the early 2020s are 1) Starshade Dedicated Mission with a roughly 1 m telescope co-launched on the same launch vehicle (to conserve cost) and 2) Starshade Rendezvous Mission with a larger starshade launched shortly after WFIRST and flown to operate with it in the latter part of the mission (including an extended mission). A variety of options are available within the two categories, with larger starshades (for larger telescopes) improving IWA and HZ access, and longer mission durations allowing more observations to establish orbits and disentangle background objects and multiple planets. 4

6 Starshade Dedicated Mission In this concept, because of its smaller mass, the telescope spacecraft provides the propulsion to retarget and control the formation. The spacecraft bus system would consist of low-cost units with extensive flight heritage. The observatory design calls for a 1.1-m diameter commercially available telescope used for Earth imaging (NextView; with four currently operational), with the main modification being the addition of a lightweight sunshade. The telescope has a conventional instrument package that includes the planet camera, an integral field spectrograph (IFS), and a guide camera. The starshade is 26 m in diameter and flies at a separation of 26 Mm. The overall cost is estimated to be $1.1B in FY 18 dollars [6]. Naturally, other designs with different apertures, formation flying separations, and fuel constraints are possible (e.g., up to 1.5 m) with corresponding increases in science return (and cost). Starshade Rendezvous Mission This concept envisions a starshade launched shortly after WFIRST and rendezvousing with it at L2 [5]. It is the current reference mission for starshade science and part of an ongoing probe class study and is the focus of technology development. NASA headquarters has directed the WFIRST project to accommodate the needed hardware and software required to make it operational with a starshade, including a starshade acquisition camera and an RF system for inter-spacecraft communication. An interface requirements document is being developed and the starshade ready requirements were included as part of the successful WFIRST Systems Requirements Review (SRR) in February of The costs associated with the needed accommodations through 2020 are borne by the WFIRST project; pending a decadal recommendation, later costs would be carried by the starshade project. The 2015 Aerospace Corporation-validated cost estimate is $630 M for the starshade+spacecraft [5]. The Coronagraph Instrument (CGI) on WFIRST will act as the focal plane instrument for the starshade portion as well. When used with the starshade, all of the coronagraph elements are removed and starshade-specific filters are used for both imaging and spectroscopy with the integral field spectrograph (IFS). The low-order wavefront sensing camera is re-purposed for starshade lateral sensing used in the formation flying control system. (All formation keeping and sky slewing maneuvers are done by the starshade; the sensed position is transmitted to the starshade view via an S band RF link.) The starshade program would use 9% of telescope time. A starshade rendezvous mission is not without programmatic, cost, and technical risks. Chief among them is the reliability and lifetime of the CGI, which is under study for risk mitigation. 6. CONCLUSION A Starshade Probe-Class mission with a small telescope (1 to 2.4 m-class size) is the fastest and lowest-cost path to the space-based direct imaging discovery and spectroscopy of planets around the Sun s nearest neighbors. Plans are in place for starshade to be ready for launch in the mid- 2020s, with a cost cap of $2B for the Dedicated Mission and $1B for the Rendezvous Mission. Focusing on our nearest one to two dozen sun-like neighbors, a starshade mission can be the first to discover and identify true Earth analogs within the context of their host planetary systems. References: [1] Anglada-Escudé, G., et al. Nature, 536, 437, 2016; [2] Gillon, M. et al. Nature, 542, 456, 2017; [3] Dittmann, J. et al. Nature, 544, 333, 2017; [4] X. Bonfils et al. A&A, in press; [5] Seager, S. et al. Exo-S Starshade Probe Report 1, 2015; [6] Mamajek, E. et al. Starshade Probe Study Update 1 report, 2018; [7] Karkoschka, E. Icarus, 111, 174, 1994; [8] Cahoy, K. et al. ApJ, 724, 189, 2010; [9] Savransky, D. et al., ascl: , scale- stdt/ 5