The TOLIMAN mission: A low-cost space telescope for high precision narrow-angle astrometry

Abstract

The TOLIMAN project is engaged with the construction, launch and operation of a low-cost space telescope of unorthodox optical design. Its primary science goal targets an exhaustive search for temperate-orbit rocky planets around either star in the alpha Centauri AB binary within our nearest-neighbor star system. Despite their favorable proximity and brightness, the detection of terrestrial exoplanets around such nearby Sun-like stars remains problematic for contemporary instrumental approaches. By performing narrow-angle astrometric monitoring of binary stars at extreme precision, any exoplanets will betray their presence by way of gravitationally-induced perturbations on the binary orbit. Recovery of this signal is challenging for it amounts to only a few microarcseconds of angular deflection (at best), and so is normally thought to require a large (meter-class) instrument. By implementing an innovative optical and signal encoding architecture, the TOLIMAN space telescope aims to recover such signals with a telescope aperture of only 12.5cm. This paper gives an overview of key features of the mission; in particular the concepts underlying the optics to enable image registration at the extreme levels of precision required. An outline is also provided, sketching further mission components and systems incorporated into the 16U CubeSat spacecraft bus in which the science payload is housed - all of which are now under construction.

Figures (7)

• Figure 1: Stellar demographics and expected planetary populations within 10 pc. Left: Number of stars by spectral typeReyl2021. Right: Expected number of planets (gray) and habitable planets (green) by host star spectral type, calculated using median occurrence rates from Figure \ref{['fig:planet_num_estimation']}.
• Figure 2: Visual summary of planet occurrence rates by stellar class from selected literature. Gray bars show median literature values for each stellar class. Individual markers represent previous studies, with error bars indicating reported uncertainties$\dagger$ in occurrence estimates. Red shaded regions denote the range of stellar classes to which each estimate applies. Marker colors correspond to publication year, and some markers are horizontally offset for visual clarity. Left: Average number of habitable$\ddagger$ planets per star. Right: Average total number$\nmid$ of planets per star. $\dagger$ Refer to individual studies for specific uncertainty measures. $\ddagger$ Refer to individual studies for definitions of the habitable zone. $\nmid$ See individual studies for sensitivity limits and corrections.
• Figure 3: The strength of indirect detection signals from an Earth-like exoplanet as a function of stellar effective temperature. Bands encompassing main sequence spectral classes A through M color coded by temperature are indicated at the top. Here, Earth-like is defined as a 1-Earth mass object in an orbit receiving irradiance equivalent to 1 AU from our Sun (the solar constant). Radial Velocity signals (solid blue line) are seen to exhibit a rapid rise in signal strength (given on the right-hand abscissa) for cooler, M dwarf systems where they become detectable with present sub-meter per second RV precision. On the other hand, astrometric deflection signals (colored diamond symbols) rise with host star temperature, and decline linearly with distance from values of up to several microarcseconds for the nearest stars. For convenience the plot also indicates the size of the temperate orbit modeled (in AU, left abscissa) and its corresponding orbital period (in years, also left abscissa).
• Figure 4: The required exposure time for an ideal 125 mm circular aperture telescope to detect given levels of astrometric signal as a function of magnitude where the only noise processes considered is photon shot noise. The lower boundary of each shaded region shows the required integration time for an ideal Airy disk PSF, while the upper boundary shows the time for the TOLIMAN PSF. The vertical lines correspond to the apparent V-band magnitudes of $\upalpha$ Centauri A & B, while red highlighting 1 $\mu$as corresponds to the smallest astrometric signal for any habitable zone Earth-mass object in this system.
• Figure 5: Left: A depiction of the circular Toliman telescope entrance pupil (including secondary and spider obstructions) together with the final optimized diffractive pupil binary-valued phase pattern. Middle: The simulated central region of the resulting PSF. Right: A simulation of the science field of $\upalpha$ Centauri binary at the appropriate separation and contrast ratio for A & B components.