The list of Investigators does not match the STScI records for this program. 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VIOLATED DURING AUTOMATIC MERGING MERGING RULE VIOLATED DURING AUTOMATIC MERGING MERGING RULE VIOLATED DURING AUTOMATIC MERGING MERGING RULE VIOLATED DURING AUTOMATIC MERGING MERGING RULE VIOLATED DURING AUTOMATIC MERGING MERGING RULE VIOLATED DURING AUTOMATIC MERGING MERGING RULE VIOLATED DURING AUTOMATIC MERGING MERGING RULE VIOLATED DURING AUTOMATIC MERGING MERGING RULE VIOLATED DURING AUTOMATIC MERGING MERGING RULE VIOLATED DURING AUTOMATIC MERGING MERGING RULE VIOLATED DURING AUTOMATIC MERGING This submitted Phase II uses available-but-unsupported observing modes (WFC3/IR G141 on GRISM512 array with SPARS10 sampling sequence). We submit this Phase II with a pending Major Change request (Number 93549, submitted 24 Apr 2026) which asks for permission from STScI to use these modes, per correspondence with the Program Coordinator and Contact Scientist. 145726141 ----- Attempting Submission 1 (Wed Apr 29 21:25:12 GMT 2026) ----- Investigator Mismatch: We include an additional co-I (Dr. Hannah Wakeford) on this submitted Phase II which will not match STScI records of investigators on this program. Hannah Wakeford was approved to be added to JWST-GO 12237 following a JWST Investigator Change Request (Number 19, approved 15 Apr 2026). As JWST-GO 12237 and HST-GO 18244 are a joint observing program, we include Hannah Wakeford on HST-GO 18244 as an extension of her inclusion in JWST-GO 12237. Orbital Visibility Overrun: accounted for by the dummy exposures included at the end of each orbit, which should not be executed in practice and therefore will not incur any overrun. Automatic Merging: also reported in the successfully-executed Visit 19 of HST-GO 13665 which this program duplicates. We welcome feedback as to whether this error will affect the data quality of our observations as well as suggestions for changes if needed to prevent these errors. We propose simultaneous observations of one transit of 55 Cnc e through JWST NIRISS/SOSS and HST WFC3/IR G141 to resolve inconsistencies in its atmospheric spectrum. JWST dayside emission studies have claimed detection of a high mean molecular weight (mmw) atmosphere rich in CO/CO2, at odds with HST transmission study claims of a low mmw atmosphere rich in H2 and HCN. Systematic differences between these instruments may be to blame, complicating efforts to interpret exoplanet atmospheres through multi-observatory studies. However, 55 Cnc e is a tidally-locked lava world with evidence for dayside variability, possibily originating from volcanic super-plumes or cloud condensation cycles. As a result, the contrasting results between JWST and HST may be astrophysical, and could yield the first evidence of spatial atmosphere variations due to extreme thermal and wind gradients, or the first evidence of transmission variability due to atmospheric and/or geologic activity. Our study leverages the observing power of both JWST and HST to confidently distinguish between all three of these scenarios, definitively revealing the true nature of this standout tidally-locked ultra-short-period super-Earth. We will observe the transiting lava world 55 Cnc e using the WFC3/IR G141. Our observation will consist of 4 HST orbits scheduled in a contiguous block. These observations should be scheduled to avoid the SAA and are phase constrained such that one HST orbit occurs during 55 Cnc e's transit, with two orbits pre-transit and one post-transit to measure the stellar baseline flux as well as characterize HST systematics so they may be corrected for. These observations are phase-constrained to start when 55 Cnc e is between orbital phases 0.7869-0.8278. These constraints are designed to capture one full orbit of in-transit flux while also maximizing the size of the starting window to facilitate scheduling. Per JWST-GO 12237, these observations must also be taken simultaneously with Visit 1 of JWST-GO 12237, which is phase-constrained to start slightly after HST-GO 18244 Visit 1 begins to account for the differences between HST and JWST detector settling timescales. The simultaneity of these observations will enable us to benchmark the performance of the new JWST NIRISS/SOSS multistripe readout mode against the performance of HST WFC3/IR G141, and detect or rule out spatial and/or temporal variability in the atmosphere of 55 Cnc e. Some adjustments have been made to the observing strategy of Visit 19 to accommodate the restrictions of reduced gyro mode. In particular, one iteration has been trimmed from the G141 spatial scans in Orbits 2, 3, and 4 to prevent orbital visibility overruns that would otherwise occur as a result of the increased time to reacquire the guide star in RGM. We have also adjusted the phase constraints based on updated ephemerides information and based on the requirements of our program to maximize in-transit flux. All four orbits include a "dummy exposure" at the end of the orbit, which use a different sampling sequence than our science exposures and which would incur orbital visibility overruns if executed. These exposures are not to be taken in practice and exist solely to force APT to trigger a buffer dump of the WFC3/IR during occultation of the target star, when it will have no impact on observations. We do request that the scheduled buffer dump occur in occultation as shown in the Orbit Planner. V. Abby Boehm (PI) is an expert in data reduction for time-series observations of transiting exoplanets. Boehm has extensive experience with both JWST and HST WFC3 exoplanet studies and has worked on the development of data reduction pipelines for both observatories. Dr. Nikole Lewis (US Admin) is an expert in JWST operations for high-precision time-series observations as well as exoplanet atmospheric theory. She has led and contributed to numerous observing programs that aim to characterize the atmospheres of exoplanets. Katy Chubb is an expert in atmospheric retrievals and opacity data. Eva-Maria Ahrer is an expert in observations of exoplanet atmospheres using transmission spectroscopy and has been PI and Co-I of JWST programs. Ailsa Campbell is experienced in data reduction of HST WFC3 time series observations of transiting exoplanets. Beatriz Campos Estrada has expertise in modelling planetary atmospheres, including testing theories of atmospheric escape, self-consistent forward modelling and retrievals. Richard Chatterjee is an expert in atmospheric escape modelling. Cinta Vidante has experience in stellar activity and transmission spectroscopy of exoplanets. Lili Alderson has extensive experience in observational analysis and interpretation of exoplanet spectra. Harrison Nicholls specializes in temperature and composition of atmospheres on lava planets with coupled interior-atmosphere evolutionary models. Elijah Mullens is an expert in exoplanet atmospheric modeling and retrievals. He has conducted multiple analyses of JWST transmission and emission spectra with a focus on characterizing disequilibrium chemistry and aerosol properties. He is the PI of two JWST Cycle 4 proposals (GO-7686, GO-8309). Hannah Wakeford (UK/ESA Admin) is an expert in transiting exoplanet observations with Hubble and JWST. Exoplanets and Exoplanet Formation 6 false false false false false false false true SECONDARY JWST 12237 false AVAILABLE STAR EXTRA-SOLAR PLANET EXTRA-SOLAR PLANETARY SYSTEM K V-IV This star is in a binary system with the second component (55 Cancri B) being an M4 dwarf star with V-magnitude 13, separated by 1.4 arcminutes. Both components share proper motion. The star 53 Cancri is also nearby at 4.6 arcminutes away, with similar V-magnitude (6.3) to our target star. 55-CNC-A GJ-324 J2000 J2000 WFC3/IR Spectroscopic No No G141 /private/var/folders/cd/zzjn_8xj19xbpfq8mj76mgqc0000gp/T/OPVisit6252407576706683133.xml 1777497008040 Phase windows are not calculated when the gap between schedulable intervals is less than 1 days, assuming no scheduling restrictions for constraint Phase 0.787/0.828. The scheduling data was generated by CASM 34.0. CASM was run in Phase II mode. 45 Per JWST-GO 12237, this observation is a duplication of Visit 19 in HST-GO 13665 and is requested to be taken simultaneously with Visit 1 of JWST-GO 12237. We require all four orbits in this visit to be taken contiguously with no interruptions, and to be scheduled to avoid the SAA. These observations are phase-constrained to begin when 55 Cnc e is between orbital phases 0.7869-0.8278 to ensure that Orbit 3 captures the maximal amount of in-transit flux. Some adjustments have been made to the observing strategy of Visit 19 to accommodate the restrictions of reduced gyro mode. In particular, one iteration has been trimmed from each 55-CNC-e Science exposure in Orbits 2, 3, and 4 to prevent orbital visibility overruns that would otherwise occur as a result of the increased time to reacquire the guide star in RGM. All four orbits include a "dummy exposure" at the end of the orbit. These exposures, labelled "Buffer Dump Trigger", contain a RAPID exposure containing the minimum number of samples needed to trigger a buffer dump to occur during occultation of the target. These exposures would incur 17 s of orbital visibility overrun in Orbit 1, and 144 s of overrun in Orbits 2, 3, and 4. In execution, we do not request these RAPID exposures actually be taken. We only include these to trigger APT to schedule a buffer dump during occultation. We do request that the in-occultation buffer dumps occur as scheduled to minimize the impact of the buffer dumps on the observations. (Tic-Visit :proposal-id 18244 :visit-id "01" :proposal-version -1 :trans-version "20250929" :date "29-Apr-2026 17:09:58.0" :trans-errors '(:WARNINGS 141 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "30" :si-bright-earth-limb-angle 20.0 :data-volume 2331136 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18244 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Obset :proposal-id 18244 :visit-id "01" :obset-id "A1" :pcs-mode "FGS" ) (Tic-Obset :proposal-id 18244 :visit-id "01" :obset-id "B1" :pcs-mode "FGS" ) (Tic-Obset :proposal-id 18244 :visit-id "01" :obset-id "C1" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18244 :visit-id "01" :orbit-number 1 :pcs-time 301 :si-vis-utilization 2580 :al-overhead-at-end 0 :internal-dur-at-end 752 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2864 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18244 :visit-id "01" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2628 :al-overhead-at-end 0 :internal-dur-at-end 747 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2864 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18244 :visit-id "01" :orbit-number 3 :pcs-time 380 :si-vis-utilization 2628 :al-overhead-at-end 0 :internal-dur-at-end 747 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2864 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18244 :visit-id "01" :orbit-number 4 :pcs-time 380 :si-vis-utilization 2628 :al-overhead-at-end 0 :internal-dur-at-end 717 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2864 :pcs-gap 0 ) true 4 4 This is a dummy exposure that is not to be taken in execution. It is included here to force APT to trigger a WFC3/IR buffer dump during the occultation of the target star, so as to prevent loss of observing time due to buffer dumps. In practice, we request that the in-occultation buffer dump occur as scheduled but that this exposure be skipped. This is a dummy exposure that is not to be taken in execution. It is included here to force APT to trigger a WFC3/IR buffer dump during the occultation of the target star, so as to prevent loss of observing time due to buffer dumps. In practice, we request that the in-occultation buffer dump occur as scheduled but that this exposure be skipped. This is a dummy exposure that is not to be taken in execution. It is included here to force APT to trigger a WFC3/IR buffer dump during the occultation of the target star, so as to prevent loss of observing time due to buffer dumps. In practice, we request that the in-occultation buffer dump occur as scheduled but that this exposure be skipped. This is a dummy exposure that is not to be taken in execution. It is included here to force APT to trigger a WFC3/IR buffer dump during the occultation of the target star, so as to prevent loss of observing time due to buffer dumps. In practice, we request that the in-occultation buffer dump occur as scheduled but that this exposure be skipped.