Oral presentations and abstracts
Session assets
Summary: Studying the atmospheres of a statistically significant number of rocky, terrestrial exoplanets -- including the search for habitable and potentially inhabited planets -- is one of the major goals of exoplanetary science and possibly the most challenging question in 21st century astrophysics. However, despite being at the top of the agenda of all major space agencies and ground-based observatories, none of the currently planned projects or missions worldwide has the technical capabilities to achieve this goal. In this contribution we present new results from the LIFE Mission initiative, which addresses this issue by investigating the scientific potential and technological challenges of an ambitious mission employing a formation-flying nulling interferometer in space working at mid-infrared wavelengths [1,2,3]. We will focus on new yield estimates and the release of our simulator software as well as improvements on our input catalog. Advances in our knowledge of the exoplanet population as well as significant progress in relevant technologies justify the need, but also the feasibility for a future mission like LIFE to investigate one of the most fundamental questions of mankind: How unique is the phenomenon we call life in the universe?
Artist's impression of the LIFE concept.
Context: One of the long-term objectives of exoplanet research is the investigation of the atmospheric properties for a large number (~100) of terrestrial exoplanets. This is partially driven by the idea to search for and identify potential biosignatures. But such a statistically significant dataset is - in a more general sense - invaluable for understanding the diversity of planetary bodies. While exoplanet science is omnipresent on the roadmaps of all major space agencies and ground-based observatories and first steps in this direction will be taken in the coming 10-15 years with funded or selected ground- and space-based projects and missions, none of them will be able to deliver such a comprehensive and consistent, big data set. An alternative to the mainly discussed large space-based coronographic missions or the starshade concept is to separate the light emitted by the planet from that of its host star by means of an interferometer. In [4] for example we showed that Proxima Centauri b is a prefectly suited target for a space-based nulling interferometer with relatively small apertures.
LIFE is a project initiated in Europe with the goal to consolidate various efforts and define a roadmap that eventually leads to the launch of a large, space-based MIR nulling interferometer. This mission should be able to investigate the atmospheric properties of a large sample of (primarily) terrestrial exoplanets. Centered around clear and ambitious scientific objectives the project will define the relevant science and technical requirements. The status of key technologies will be re-assessed and further technology development will be coordinated. LIFE is based on the heritage of ESA/Darwin and NASA/TPF-I, but significant advances in our understanding of exoplanets and newly available technologies will be taken into account in the LIFE mission concept.
New Results and Progress: In a previously presented work [5], we used Monte Carlo simulations to demonstrate that a MIR space-based nulling interferometer like LIFE, could yield at least as many exoplanet detections as a large, single aperture optical/NIR telescope. Here we will present an elaborate update on this first study. A key aspect that we have investigated more closely is the specific treatment of stellar leakage and exozodical light in our simulations. We also had a critical look at the stellar input sample and its properties with a specific focus on multiplicity.
The details and exact number of planets depend on the assumed technical specifications and the underlying exoplanet populations, but from an exoplanet science perspective such an interferometer should be considered an attractive mission concept, at least complementary if not superior to an optical/NIR mission.
We will present our newest data simulator that incorporates various telescope sizes and a new noise model that takes into account all astrophysical noise sources. This enables us to systematically study our mission requirements in order to optimize our observing strategy.
As most detected planets will be warmer than Earth, going as short as 3 μm seems useful; at the red end 25 μm seems sufficient. This wavelength range features absorption bands of CO2, H2O, O3, CH4, (N2)2, and N2O and also contains windows to probe surface emission. The spectral resolution (R ~ 20-100) is very likely to be driven by the need to avoid line contamination of certain molecules such as N2O and CO2 around 4.15 μm, as well as CH4 and also N2O and H2O between 7.7 and 8 μm.
In another submission to this conference (Konrad et al. 2020) we discuss more details on our progress in spectral retrieval.
Future steps: Our analysis also shows that getting a better handle on the overall planet statistics is crucial for planning larger future missions. We are therefore working on a detailed simulation of the impact on scheduling for the survey and characterisation phases of the LIFE mission. In this context we are also currently investigating modern machine learning methods that crucial to scale up front to end simulations of the full LIFE survey. This in turn will not only inform aforementioned scheduling consideration but also help to define sensitivity, wavelength coverage and spectral resolution requirements on the technology side.
References: [1] Quanz, S. P., Kammerer, J., Defrère, D., et al. 2018, Optical and Infrared Interferometry and Imaging VI, 10701,107011I. [2] Defrère, D., Léger, A., Absil, O., et al. 2018, Experimental Astronomy, 46, 543. [3] Quanz,S. P.,et al. 2019. , arXiv e-prints arXiv:1908.01316. [4] Defrère, D., Léger, A., Absil, O., et al. 2018, Optical and Infrared Interferometry and Imaging VI, 10701,107011H. [5] Kammerer, J., & Quanz, S. P. 2018, A&A, 609, A4
How to cite: Angerhausen, D., Quanz, S., and initiative, T. L.: The LIFE mission: a mid-infrared space interferometer to study the diversity of terrestrial exoplanets, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-591, https://doi.org/10.5194/epsc2020-591, 2020.
Summary
LIFE is a space-based mid-infrared (MIR) nulling interferometer proposed in Quanz et al. (2018). We use a Bayesian retrieval framework to analyze how the information on the atmospheric structure extractable from MIR emission spectra of terrestrial exoplanets change with the covered wavelength range, signal to noise ratio (SNR) and spectral resolution (R). In combination with the science goals for the LIFE mission, these simulations will provide us with first quantitative estimates of key technical requirements needed for such a telescope.
Context
A long-term objective of exoplanet research is to investigate the atmospheric properties of small and rocky exoplanets. A driving force behind this effort is the potential identification of habitable worlds. This data will improve our understanding of the diversity of extra-solar planetary bodies. Currently planned space- and ground-based telescopes are unlikely to be sensitive enough to characterize a large number of such atmospheres.
Future space missions (e.g. NASA's LUVOIR [a] and HabEx [b] concepts), capable of characterizing exoplanet atmospheres in reflected light (in the optical and near-infrared (NIR) wavelength range), have been proposed. Complementary to these efforts, the LIFE mission proposed in Quanz et al. (2018), which is discussed in more detail in another submission to this conference by Angerhausen et al., targets the mid-infrared (MIR) thermal emission of exoplanets using a nulling interferometer. This wavelength range is of particular interest since it allows us to probe for atmospheric molecules not accessible in the optical or NIR range such as the biosignatures CH4 and N2O as well as CO. Furthermore, MIR emission observations, in contrast to optical and NIR observations, are capable of accurately probing the temperature structure and the radius of exoplanets (Quanz et al. 2019). Both are hardly constrained in reflected light observations.
Methods
Our approach is based on the retrieval of simulated mock observations of Earth-twin exoplanets around solar-type stars. We generate the emission spectrum of an Earth twin using the 1D radiative transfer tool petitRADTRANS (Mollière et al., 2019). The model takes the surface gravity, the pressure-temperature (PT) profile and the atmospheric gas composition as input. We parametrize the standard Earth’s PT profile with a 4th order polynomial and consider the line Absorption from H2O, CO2, CH4, O3, N2O and CO as well as collision-induced absorption from N2 and O2.
Then, we use the LifeSIM (Ottiger et al. in prep.) tool to estimate the wavelength-dependent SNR expected for observations with a LIFE-like telescope, considering noise sources from stellar leakage, local zodiacal dust, and exozodiacal dust. We consider an Earth twin located at a distance of 10 pc from the Sun around a G2 star with 3 exozodis. We generate mock observations for the following grid of wavelength ranges, Rs and SNRs:
- Range = 3-20 μm, 6-17 μm
- R = 20, 35, 50
- SNR = 5, 10, 15
Our retrieval mechanism is built upon the Multinest algorithm (Feroz et al., 2009). Given the Earth twin input spectrum and treating the LifeSIM noise as uncertainty on the spectrum, we retrieve the PT profile, the mass, the radius, the surface pressure as well as the abundances of the different atmospheric species. Performing a retrieval study of the spectra taking into account all possible combinations of the specified ranges in wavelength, R and SNR, provides first estimates for the technical requirements we need to meet to characterize Earth-like atmospheres.
Results
A preliminary result, and a proof of concept from testing of our retrieval framework, is shown in Figures 1 and 2. The input for the retrieval is an Earth-twin spectrum (range = 3 - 25 μm, R = 1000, SNR = 50) seen at a distance of 10 pc from the Sun. Please note that for the results given below, only the planet’s photon noise was considered and not yet additional noise sources.
Figure 1 shows that the PT profile parameters, as well as the surface temperature, are accurately retrieved by our retrieval suite. Figure 2 gives the retrieved posteriors for the planet’s radius, mass and atmospheric abundances. The histogram in the top-right corner indicates the retrieved absolute abundance of CO2. We give all other abundances relative to the CO2 abundance. Additionally, our retrieval framework constrains the planet's radius with high fidelity.
These first results demonstrate that our retrieval suite provides reliable results. In-depth analyses of the results for the grid mentioned above are currently ongoing and will be presented at the conference. In addition, a comparison with literature results from simulations of reflected light observations will be discussed.
References
Feroz, F., et al., 2009, MNRAS, 398(4):1601–1614
Mollière, P., et al., 2019, A&A, 627:A67
Quanz, S. P., et al., 2018, Proc. SPIE, 107011I
Quanz, S. P. et al., 2019, arXiv e-prints arXiv:1908.01316
Websites
[a] https://www.luvoirtelescope.org
[b] https://www.jpl.nasa.gov/habex/
How to cite: Konrad, B. S., Alei, E., and Quanz, S. P. and the LIFE inititiative: Atmospheric Retrieval Sensitivity Analysis for an Earth-Twin in the Future LIFE Mission, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-650, https://doi.org/10.5194/epsc2020-650, 2020.
Abstract
Discovering just a single exoplanet with signs of life would be an event of huge scientific and cultural significance. Yet, that single discovery may not shed light on an even more fundamental question: how common is life in the Galaxy? This can be parameterized by the fL term in the famous Drake equation [1]. Biomarker molecular spectral signatures may indicate the presence of life on an exoplanet. We propose a mission concept called DRAKE (Dedicated Research for Advancing Knowledge of Exobiology) to survey a statistically significant number of M-dwarf habitable zone terrestrial planets using transit spectroscopy. We explore this concept through preliminary observatory and mission simulations.
1. Introduction
Due to sensitivity considerations such a mission would only be viable for M-dwarf planets, which is a disadvantage compared to direct-imaging approaches. In addition the available sample size is reduced through the transit probability (~2%). However transit spectroscopy is a relatively seasoned method with several previous and approved future space-borne missions. In addition since M-dwarfs are ~75% of all stars, and since the frequency of Earth-sized planets in the habitable zones of M-dwarfs and Sun-like stars is about the same [2][3], it may be the case that such M-dwarf planets represent the Galactic norm not the exception. Currently there are only 18 known transiting terrestrial planets within the optimistic habitable zone boundaries of M-dwarf stars (http://phl.upr.edu/projects/habitable-exoplanets-catalog). However in the coming years this number is likely to increase considerably due to new discoveries by TESS, PLATO, Cheops and ground-based transit surveys, making a transit spectroscopy-based survey potentially viable.
2. Sample size
What sample size would be needed to constrain fL to within a given margin of error? Figure 1 shows the results of Monte Carlo simulations that calculate the margin of error, e, for 95% confidence level at different sample sizes, Nsamp. This is obtained through simulation of an overall population of size Npop and assumes a true value for fL between 0 and 1.0. e is half of the 95% confidence limit obtained for the distribution of fL sample estimates over 1000 samples, and is maximal (emax) when fL = 0.5. Figure 1 shows that the curves of emax vs Nsamp converge at higher Npop. At Npop = 106, we find that 50 and 100 planet samples could constrain fL to ± 14% and 9.6% respectively at 95% confidence.
Figure 1. emax vs sample size for different population sizes. Dots are the mean over 10 repeats of 1000 realization Monte Carlo simulations, and error bars are the standard deviation.
3. Observatory design
The concept calls for a telescope operating from the L2 Sun-Earth Lagrange point, with an infra-red spectrometer instrument. The near- and mid- infra-red contains spectral signatures for major molecules occuring in planetary secondary atmospheres such as H2O, CO2, CH4 and NH3, as well as biosignatures from O2 and O3. Ro-vibrational band envelopes produce spectral features that generally broaden at longer wavelengths. Previous work [4] has shown that most features from 5-10 μm can be characterized with R ~ 30, whereas narrower features at shorter wavelength require R ~ 100. Two prototype instruments are explored here: 1) 0.6-12 μm in two channels, Ch0 (0.6-5 μm at R = 100) and Ch1 (5-12 μm at R = 30), and 2) 0.7-10.5 μm in two channels, Ch0 (0.7-5 μm, R = 100-50), and Ch1 (5-10.5 μm, R = 30-15). The varying R power and reduced spectral range of the latter design increases the minimum atmospheric SNR (Figure 2). The ability to identify spectral features reliably using the variable R in prototype 2 will require further study using spectral retrieval methods.
Figure 2. SNR1(λ), the atmospheric SNR in 1 transit for an Earth twin around an M0 star at 10 pc distance with a 10 m telescope. Red line: prototype 1. Blue line: protoype 2. The green line shows the varying R power across the band for prototype 2.
4. Simulations
Example results from initial simulations are shown in Figure 3. The method first generates a randomized simulated population of transiting Earth-sized and super-Earth-sized planets in the habitable zones of M-dwarfs of different subclasses (with a spatial distribution based on data from the TESS Input Catalogue Candidate Target List). These planets are assumed to be detected in order of their relative detection SNR to form samples of size Nsamp ranging from 50 to 400. We use a simple instrument model for DRAKE with primary mirror size, D, and one of the two prototype instrument designs. We assume photon noise only, and a ’box-car’ transit model. The scale height, H, is modelled for each planet assuming it has an average temperature and mean molecular weight equal to that for the Earth. This is used to estimate the size of a spectral feature, A, where A = 2(nHHRp)/Rs2 (nH = 5 or 7, Rp = planet radius, Rs = star radius). The noise on A, σA(λ), is calculated, to give an atmospheric SNR for 1 transit, SNR1(λ). The number of transits, Nt, to reach an target SNR, SNRt (= 3 or 5) is then calculated for each planet using the minimum SNR1(λ). Each planet is randomly assigned a T0 central transit time, and a scheduling algorithm is used to efficiently order the planet sample observations and obtain a total mission duration, Tmiss.
Figure 3. Prototype instrument 2 with optimistic detection criteria. Mission duration vs primary mirror size, for different sample sizes. Points with error bars show the mean and minimum-maximum range over 20 realizations. Dashed lines and shaded areas follow from power law fits.
5. Conclusions
We have shown that such a mission is achievable within a 5-20 year period under various conditions, e.g. prototype 2 completes Nsamp = 50 with a 14 m telescope in 7.5 years (assuming nH = 7, SNRt = 3). This mission concept deserves further study and development, e.g. using more detailed instrument models and advanced detection metrics such as spectral retrievals.
References
[1] Drake, F. (1961) Green Bank conference, 1961.
[2] Dressing, C.D. & Charbonneau, D. (2015) ApJ, 807, 45.
[3] Petigura, E. A., Howard, A. W. & Marcy, G. W. (2013) PNAS, 110, 19273.
[4] Sarkar, S., Stevens, S. & Griffin, M. (2019) EPSC Abstracts, 13, EPSC-DPS2019-655-1
How to cite: Sarkar, S.: The DRAKE mission: finding the frequency of life in the Galaxy, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-283, https://doi.org/10.5194/epsc2020-283, 2020.
