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.
