Can a telescope that is making a search of the life of the worlds far away depend on what a star is doing somewhere in the background?
Pandora was constructed on the basis of that awkward question, since the following step in exoplanet science is not to raise more photons. It is learning that the light is lying. Transit spectroscopy, which involves observing a planet passing in front of its star and observing the minute wavelength-by-wavelength “bite marks” which gases leave behind, has become the standard tool in studying atmospheres. But the same method is exposed to the influence of the star itself, whose spotted surface may assume forms which resemble those of planets. Pandora is meant to quantify directly that stellar contribution, and to direct the targets of bigger observatories like the James Webb Space Telescope.
The fundamental issue is called the effect of transit light source. Bright active locations and dark starspots can change the apparent depth of a transit in a manner that confounds itself as atmospheric chemistry. On most stars, these areas vary on the timescales of interest to observers, hours to days, and then a single beautiful Webb spectrum is constrained by the uncertainty in which part of the stellar surface was “backlighting” the planet at the time of transit. Practically this can give a false confidence: water signatures can be boosted or lowered and molecules which are linked to stricter stellar layers can be mistaken with gases in the atmosphere of a planet.
The strategy that Pandora uses is to keep the mission small and the observing strategy patient. Pandora instead repeats, long glances at a system to create a map of stellar behaviour than searching infrequently. The concept of the mission is based on the idea of the long-term multiwavelength observations which monitor the change in the brightness and the color of the host star as a planet transits. Observing the variability in the visible light will add to the observations of the near-infrared measurements, thus enabling Pandora to limit the coverage of the spots and the faculas, and isolate the evolving spectrum of the star to the vastly smaller atmospheric signal of the planet. The plan of operation is to visit the 10 visits per system, each time will last approximately 24 hours, approximately 12 systems in approximately one year, so the telescope will be able to observe the active regions as they turn around into sight, develop, and disappear.
The reason that cadence is important is that stellar contamination is not just an abstract nuisance, but a noise source that varies with time, and whose contribution varies with wavelength and can be dominant in the error budget of small planets. More recent techniques based on Webb data have revealed the distance reachable by clever calibration by the observer. One of them showed an epoch-based correction through back-to-back transits in the TRAPPIST-1 system and a 2.5x purification in stellar contamination in short wavelengths- transforming an organized systematic into more of a white noise, which advances with repeated measurements. Pandora has that philosophy institutionalized: quantify the behavior of the star as a first-class dataset, but not as an appendix to the planet.
It also explains why Pandora is a complementary, and not a competitor, of Webb. Webb has the power of sensitivity and spectral detail, Pandora, the power of persistence and context. Coupling helps to increase the likelihood that molecules can be attributed to the appropriate source, either star or planet, in a transit spectrum, before a field becomes overreliant on single-visit interpretations.
A mission architecture that is speedy and focused is behind the science case. The selection of Pandora occurred after NASA initial request to submit concepts of an Astrophysics Pioneer mission and NASA Goddard was the leader and Lawrence Livermore was the project manager and engineer. Blue Canyon Technologies is connected with the spacecraft bus and the continuing checkout, whereas the control of mission data processing is planned at NASA Ames, and the University of Arizona is a location where the mission operations are scheduled. What will come out is a specialized observatory: smaller than flagship telescopes, but targeted on the bottleneck that is now constraining the extent to which astronomers can describe the atmospheres of Earth-size worlds.