EarthFinder is a NASA mission probe concept study that evaluates the benefits of performing precise radial velocity measurements (PRVs) from space to detect Earth-like exoplanets. To date, all PRVs have been performed by ground-based telescopes. These observations are limited by atmospheric contamination, stellar activity, and instrument stability/calibration. These ground-based telescopes excel at detecting Jupiter-mass planets, but only because the amplitude of their radial velocity (RV) signal is so high (> 1 m/s). Earth-like exoplanets–planets about the same mass as Earth and orbiting in the Habitable Zone around their host star–only have a RV signal of about 9 cm/s. The aforementioned limitations make it exceedingly difficult to detect such a small signal amplitude, particularly from the ground. Indeed, our current technology can only detect signal amplitudes down to 1 m/s. EarthFinder’s unique approach of performing PRVs in space allows effective mitigation strategies for stellar activity and instrument stability/calibration, and completely eliminates atmospheric contamination.
Radial velocity measurements require analyzing the motion of absorption lines in spectra over time. Ideally, absorption lines only come from the star we are observing. In reality, Earth’s atmosphere introduces absorption lines into our spectra (these absorption lines are called tellurics). These absorption lines can be modeled out, but only down to a certain level. Eventually, once we get to a high enough precision, we are faced with micro-telluric lines. These lines are small in depth, but are time-varying and thus present an enormous problem in analyzing RV data. The only way to completely eliminate this problem is to take measurements above Earth’s atmosphere. Our simulations find an optimistic precision floor imposed by telluric contamination. We show that RV observations made in visible wavelengths can achieve a 1 cm/s precision, but in the near-infrared (> 700 nanometers), the limit is 30 cm/s. If it is necessary to detect Earth-like planets by using multiple wavelength bands (particularly visible and near-infrared), it will not be possible to do so from the ground.
There are many forms of stellar activity that interfere with RV measurements at varying levels, including stellar oscillations, flares, granulation, short-term activity (spots and faculae), etc. The most significant forms of activity we need to correct for are granulation and short-term activity when trying to find Earth-like planets. EarthFinder offers a wealth of benefits over ground-based telescopes in mitigating stellar activity, including extremely high signal-to-noise ratios (SNR) and resolution, broad wavelength coverage free of telluric errors, RV color measuring capabilities, and ideal sampling rates. EarthFinder’s resolution is about 150,000, and combined with unprecedented SNR (afforded by lack of atmosphere) we can perform line-by-line analysis of stellar spectra to mitigate stellar activity down to the level of a few cm/s. Regarding RV color, planetary signals are independent of color (i.e what wavelength we observe), but stellar signals do depend on wavelength. We can use this fact to uniquely isolate our planetary signals from the stellar activity. Using a simple toy model (making RV color proportional to visible stellar activity), our simulations show we can reduce the RV RMS activity by 62%, which is better than any result from ground-based telescopes that model stellar activity with the most sophisticated line-by-line analysis procedures. Lastly, EarthFinder’s susceptibility to signal aliasing is effectively nullified, since it is free of diurnal and seasonal sampling biases (and is free of right ascension and declination biases). These cadence advantages allow for a much more even sampling of the stellar activity (and planet signal), which, as indicated by our simulations, aid in the reduction of stellar activity noise.
Our RVs are only as precise as our calibration source. Further, extreme spectrograph stability is needed to achieve our goal of 3 cm/s precision. A relatively new technology, optical frequency combs, are an extremely precise calibration reference. These combs provide ideal wavelength references over a fair portion of the electromagnetic spectrum. Indeed, their use as an extremely precise calibration source has already been proven in ground-based spectrographs. The problem, though, with these combs is that they require significant power consumption. While it is not outright impossible to utilize these combs in a spacecraft, it does significantly increase the cost of the mission. While an ideal optical frequency comb does not exist yet for spacecraft, our collaborators have demonstrated a very promising path forward in the development of our idealized optical frequency comb. The combs already in existence are only a few years old, so with this rapidly developing technology, it is fully expected that we can engineer a suitable comb within the coming years.