NIRPS Science Cases
NIRPS has been designed to explore the exciting prospects offered by the M-dwarfs, focusing on three main science cases.
Mass and density measurements of transiting Earths around M dwarfs
Thousands of planets transiting nearby M-dwarfs are expected to be found in the coming years. The TESS and PLATO will be the main sources of transit candidates. Ground-based surveys such as ExTra and TRAPPIST in La Silla, NGTS and Speculoos in Paranal or Mearth will provide a continuous supply of additional targets.
Given the high fraction of M dwarfs hosting Earth-like planets inside the HZ, it is expected that the yield of habitable planets among the M dwarfs with transiting planets be equally high. Those planets will be subject of an intensive follow-up in RV and photometry, and will be primary targets for atmospheric studies with JWST. A proper interpretation of the JWST data will ultimately require a constraint on the bulk density of the planet. Knowing the planetary radius from transit depth, a mass estimate is therefore required, and this can only be obtained through near-IR RV data or, in very specific cases, Transit-Timing Variation (TTV). NIRPS is ideal to target M dwarfs, being able to provide masses for a large number of transiting planets to disentangle transiting planets from diluted background eclipsing binaries. Figure 2 illustrates the parameter space allowed by NIRPS in comparison to state-of-the-art optical spectrographs; considering that most host stars will be M dwarfs, NIRPS will allow the mass measurement of a large number of super-Earths in their HZ.
Figure 2. Simulated TESS sample of Southern (declination < 20°) planets in an insolation versus radius diagram. Planets amenable to HARPS follow-up are shown in red while those, much more numerous, amenable to NIRPS follow-up are shown in blue. NIRPS will allow the follow-up of numerous planets that are only slightly larger than Earth (1-2.5 R⊕) and that receive a comparable insolation (0.3-10 S⊕). Radius, insolation and photometric values are drawn from the  simulated set. These planets will be prime targets for atmospheric characterisation studies with JWST.
Preparing the 2030’s: an RV search for planets to image with the ELT
M dwarfs are also preferred targets for direct imaging studies to be carried out with future extreme adaptive optics (XAO) imagers on the ELTs. NIRPS will monitor a sample of the closest ~100 southern M dwarfs with the goal of finding the closest habitable worlds to the Sun (See Figures 3 and 4). As these planets are most likely members of multi-planetary systems, this survey requires a relatively large number of visits per star (100 to 200), as demonstrated by HARPS experience. Although such a program can be performed by NIRPS alone, simultaneous HARPS observations will increase the overall efficiency the telescope, as it permits to improve the photon noise budget by up to 15-40%, depending on the effective temperature of the host star. Moreover, the simultaneous use of NIRPS and HARPS can help to disentangle planetary signals from pure stellar jitter as stellar activity shows chromatic dependence whereas the planetary signal, due to the planet’s gravitational pull, is known to be achromatic. This will enhance the scientific output by filtering out false positives efficiently. GAIA astrometry and direct imaging will complement such study by detecting planets on wide orbits (>1000 days).
Figure 3. Simulated NIRPS planet survey results in the insolation/minimum planet mass plane. With the predicted NIRPS performances and realistic stellar properties we recovered 79 planets around 100 stars in 150 to 200 visits per star. The detection framework is described in . The size of each marker is proportional to the planet’s radius. The approximate ‘maximum greenhouse’ and ‘water-loss’ limits of the habitable zone are highlighted in blue (0.2 ≤ S/S⊕ ≤ 1).
Figure 4. The same simulated NIRPS planet population as shown in Fig. 3 in the projected separation/contrast plane. To compute the contrast in reflected light we assume a geometrical albedo of A=0.3 for all planets. Shaded circles represent planets that would be detected with NIRPS; detected HZ planets are highlighted in blue and detected rocky (rp < 1.5 R⊕) HZ planets highlighted in red. The planet population is compared to the expected contrast curve expected to be achieved by third generation of ELT near-IR imagers. Red diamonds show the estimated location of nearby HZ planets around M dwarfs [27,28,29].
Atmospheric characterisation of exoplanets
Transiting planets offer a unique opportunity to gather information about the composition and temperature of their atmospheres, as well as the presence of molecular species, including biosignature gases or surface atmospheric features. High-resolution transmission spectroscopy allows tracking the wavelength shift of individual narrow spectral features in the atmosphere as the planet orbits the star. As an example, HARPS observations of the Hot Jupiter HD189733 allowed to spectrally resolve the Na doublet, to measure its line contrasts and to derive the temperature at two different altitudes.
Thanks to its large spectral coverage, several spectroscopic features are present within the wavelength range of NIRPS, such as, CO, CO2, CH4, H2O in H band, but also Na, H2O in the visible domain, for instance. This plethora of molecules makes NIRPS very competitive in characterizing the atmosphere of hot-Jupiters and hot-Neptunes. In addition, by measuring the spectroscopic transit (Rossiter-McLaughlin effect) the projected spin-orbit alignment can be measured providing an important parameter linked to the formation and dynamic evolution of the system. The Rossiter-McLaughlin effect for small planets orbiting M dwarfs has never been measured; these observations will thus provide new insights into the dynamical histories of such planets.
Other science cases covered by NIRPS
While exoplanet detection and characterization will take the lion’s share of NIRPS observing time, a number of other significant science niches are foreseen for the instrument. NIRPS is expected to contribute to the dynamical studies of ultracool dwarfs in young moving groups, enabling RV measurements well into the sub-stellar regime all the way to the deuterium burning limit. These require km/s-level accuracy at nIR wavelengths. The exquisite line-spread function stability, as demanded for exoplanet detection, will enable stellar variability studies that attempt to measure minute variations in line profiles such as Doppler imaging of ultracool stars and brown dwarfs.
The simultaneous observation with HARPS and NIRPS will enable a better calibration of stellar activity during RV monitoring of Sun-like stars. Nearby G and K stars are bright enough to allow m/s-precision measurement in either optical or nIR.
Near-infrared wavelengths are the best when observing cool, red M-dwarfs, not only because their spectral energy distribution makes them more than one order of magnitude brighter in the NIR than in the visible, but because nIR stellar spectra are significantly less blended than their visible counterparts. This factor is key in allowing for a more precise line-by-line analysis[25,26] and motivates the expansion of well-round spectroscopic analysis to the nIR. The derivation of precise stellar parameters will allow us to move one step further, and obtain precise chemical abundances for key elements (such as alpha and iron-peak elements) in M dwarfs, opening new avenues for research, such as the chemical evolution of the Galaxy as monitored by its most populous inhabitant.
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