17600( 8) - 21-Nov-2025 15:10:16 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17600 Version: 8 Check-in Time: 21-Nov-2025 15:10:16 STScI Edit Number: 1 Title The Exosphere of an Venus-Like Exoplanet ------------------------------------------------------------------------------------ Type Cycle GO 31 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Shreyas Vissapragada Carnegie Institution of Washington Y CoI: Larissa Palethorpe University of Bristol N CoI: Shishir Dholakia University of Southern Queensland N CoI: Daisy Turner University of Birmingham N CoI: Dr. Annelies Mortier University of Birmingham N CoI: Prof. Ken Rice University of Edinburgh, Institute for N Astronomy CoI: Dr. Thomas G. Wilson University of Warwick N CoI: Dr. George Zhou University of Southern Queensland N ------------------------------------------------------------------------------------ Abstract We propose a 10-orbit HST/STIS Mid-Cycle program to study the exosphere of the recently discovered terrestrial exoplanet GJ 12b (TOI-6251.01). With a Venus-like radius (1.016 +/- 0.062 R_Earth) and equilibrium temperature (327 K), GJ 12b offers us a rare chance to explore important Solar System processes like the runaway greenhouse effect on an extrasolar planet. This is also the best known terrestrial planet for Lyman-alpha transit observations: GJ 12's Lyman-alpha line is likely to be intrinsically strong based on the strong stellar X-ray emission, and its close proximity (12.2 pc) and high systemic radial velocity (51.2 km/s) partially offset the (relatively weak) interstellar absorption from the stellar line core. These factors result in much higher precisions than usually achievable for exoplanet Lyman-alpha studies: with two transits of five orbits each, we expect to constrain exospheric hydrogen absorption in GJ 12b to a precision of 0.9% in the red wing of the Lyman-alpha line. At this precision we would detect a Roche-lobe-filling transit at SNR = 38.7, and we would be sensitive (at the 3-sigma level) to neutral hydrogen down to 5R_p, near the planetary exobase. By definitively confirming or ruling out exospheric hydrogen in Venus-like exoplanet, this Mid-Cycle program will significantly advance our understanding of the upper atmospheres of terrestrial exoplanets. ------------------------------------------------------------------------------------ Observations Description ------------------------ Our program will observe two transits of GJ 12b with STIS MAMA-FUV G140M 1222A. The goal is to constrain planetary absorption in the Lyman-alpha line, i.e. to observe a decrease in the strength of the stellar LyA line during the planetary transit. Each visit includes 5 consecutive orbits, and a phase constraint has been placed on the first exposure of orbit 1 such that orbit 3 is centered on the planetary transit. As the transit duration is fairly short (1 hr), our timing requirement for the first exposure is somewhat strict (30 min) but still allows for some scheduling flexibility. The first orbit for each visit includes ACQ and ACQ/PEAK exposures, and all orbits include a science exposure and a 17600( 8) - 21-Nov-2025 15:10:16 - [ 2] wavecal exposure. Reduced gyro mode would impact our program by limiting the number of transits that could be feasibly scheduled. Our primary target has a fairly long orbital period, so scheduling is already somewhat challenging. ------------------------------------------------------------------------------------ 17600( 8) - 21-Nov-2025 15:10:16 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 G-32-5 STAR, M V-IV RA=00H15M49.9205S +/- J2000 V = 12.6 +/- 0.04 0.0000274", DEC=+13D33'27.59" +/- 0.0000234" Reference Frame: ICRS Comments: Positional data from Gaia DR3, V magnitude from SIMBAD. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2016 0.042385425126215105 0.329446 0.0821938 ------------------------------------------------------------------------------------------------------------------------------------ 17600( 8) - 21-Nov-2025 15:10:16 - [ 4] Visit: 01 (GJ 12b visit 1) Visit Requirements: Period 12.76142 D AND ZERO-PHASE HJD2459497.183 On Hold Comments: Additional Comments: First transit of GJ 12b. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 G-32-5 STIS/CCD ACQ F28X50OII MIRROR 1 5 S (5 S) PHASE 0.9919888386 TO I 0.9944376256; SEQ 1-4 NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 2 G-32-5 STIS/CCD ACQ/PEA 0.2X0.05N MIRROR 1 10 S (10 K D S) ------------------------------------------------------------------------------------------------------------------------------------ 3 G-32-5 STIS/FUV TIME-TA 52X0.1 G140M 1222 BUFFER-TIME=835, WAVECAL=NO 1 1350 S -MAMA G (1350 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 WAVE STIS/FUV ACCUM 52X0.1 G140M 1222 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 5 G-32-5 STIS/FUV TIME-TA 52X0.1 G140M 1222 BUFFER-TIME=1357, WAVECAL=NO 1 2600 S SEQ 5-6 NON-INT -MAMA G (2600 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 WAVE STIS/FUV ACCUM 52X0.1 G140M 1222 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 7 G-32-5 STIS/FUV TIME-TA 52X0.1 G140M 1222 BUFFER-TIME=1357, WAVECAL=NO 1 2600 S SEQ 7-8 NON-INT -MAMA G (2600 S) ------------------------------------------------------------------------------------------------------------------------------------ 8 WAVE STIS/FUV ACCUM 52X0.1 G140M 1222 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 9 G-32-5 STIS/FUV TIME-TA 52X0.1 G140M 1222 BUFFER-TIME=1357, WAVECAL=NO 1 2600 S SEQ 9-10 NON-INT -MAMA G (2600 S) ------------------------------------------------------------------------------------------------------------------------------------ 10 WAVE STIS/FUV ACCUM 52X0.1 G140M 1222 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 11 G-32-5 STIS/FUV TIME-TA 52X0.1 G140M 1222 BUFFER-TIME=1357, WAVECAL=NO 1 2600 S SEQ 11-12 NON-INT -MAMA G (2600 S) ------------------------------------------------------------------------------------------------------------------------------------ 12 WAVE STIS/FUV ACCUM 52X0.1 G140M 1222 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 17600( 8) - 21-Nov-2025 15:10:16 - [ 5] Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ G-32-5 1 STIS/CCD ACQ F28X50OI MIRROR none none none none 1 N/A II G-32-5 2 STIS/CCD ACQ/PEA 0.2X0.05 MIRROR none none none none 1 N/A K ND G-32-5 3 STIS/FUV TIME-TA 52X0.1 G140M 1222 none none none none 1 N/A -MAMA G WAVE 4 STIS/FUV ACCUM 52X0.1 G140M 1222 none none none none 1 N/A -MAMA G-32-5 5 STIS/FUV TIME-TA 52X0.1 G140M 1222 none none none none 2 N/A -MAMA G WAVE 6 STIS/FUV ACCUM 52X0.1 G140M 1222 none none none none 2 N/A -MAMA G-32-5 7 STIS/FUV TIME-TA 52X0.1 G140M 1222 none none none none 3 N/A -MAMA G WAVE 8 STIS/FUV ACCUM 52X0.1 G140M 1222 none none none none 3 N/A -MAMA G-32-5 9 STIS/FUV TIME-TA 52X0.1 G140M 1222 none none none none 4 N/A -MAMA G WAVE 10 STIS/FUV ACCUM 52X0.1 G140M 1222 none none none none 4 N/A -MAMA G-32-5 11 STIS/FUV TIME-TA 52X0.1 G140M 1222 none none none none 5 N/A -MAMA G WAVE 12 STIS/FUV ACCUM 52X0.1 G140M 1222 none none none none 5 N/A -MAMA 17600( 8) - 21-Nov-2025 15:10:16 - [ 6] Visit: 02 (GJ 12b visit 2) Visit Requirements: Period 12.76142 D AND ZERO-PHASE HJD2459497.183 On Hold Comments: Additional Comments: Second transit of GJ 12b. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 G-32-5 STIS/CCD ACQ F28X50OII MIRROR 1 5 S (5 S) PHASE 0.9876680898 TO I 0.9893006145; SEQ 1-4 NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 2 G-32-5 STIS/CCD ACQ/PEA 0.2X0.05N MIRROR 1 10 S (10 K D S) ------------------------------------------------------------------------------------------------------------------------------------ 3 G-32-5 STIS/FUV TIME-TA 52X0.1 G140M 1222 BUFFER-TIME=835, WAVECAL=NO 1 1350 S -MAMA G (1350 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 WAVE STIS/FUV ACCUM 52X0.1 G140M 1222 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 5 G-32-5 STIS/FUV TIME-TA 52X0.1 G140M 1222 BUFFER-TIME=1357, WAVECAL=NO 1 2600 S SEQ 5-6 NON-INT -MAMA G (2600 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 WAVE STIS/FUV ACCUM 52X0.1 G140M 1222 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 7 G-32-5 STIS/FUV TIME-TA 52X0.1 G140M 1222 BUFFER-TIME=1357, WAVECAL=NO 1 2600 S SEQ 7-8 NON-INT -MAMA G (2600 S) ------------------------------------------------------------------------------------------------------------------------------------ 8 WAVE STIS/FUV ACCUM 52X0.1 G140M 1222 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 9 G-32-5 STIS/FUV TIME-TA 52X0.1 G140M 1222 BUFFER-TIME=1357, WAVECAL=NO 1 2600 S SEQ 9-10 NON-INT -MAMA G (2600 S) ------------------------------------------------------------------------------------------------------------------------------------ 10 WAVE STIS/FUV ACCUM 52X0.1 G140M 1222 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 11 G-32-5 STIS/FUV TIME-TA 52X0.1 G140M 1222 BUFFER-TIME=1357, WAVECAL=NO 1 2600 S SEQ 11-12 NON-INT -MAMA G (2600 S) ------------------------------------------------------------------------------------------------------------------------------------ 12 WAVE STIS/FUV ACCUM 52X0.1 G140M 1222 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 17600( 8) - 21-Nov-2025 15:10:16 - [ 7] Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ G-32-5 1 STIS/CCD ACQ F28X50OI MIRROR none none none none 1 N/A II G-32-5 2 STIS/CCD ACQ/PEA 0.2X0.05 MIRROR none none none none 1 N/A K ND G-32-5 3 STIS/FUV TIME-TA 52X0.1 G140M 1222 none none none none 1 N/A -MAMA G WAVE 4 STIS/FUV ACCUM 52X0.1 G140M 1222 none none none none 1 N/A -MAMA G-32-5 5 STIS/FUV TIME-TA 52X0.1 G140M 1222 none none none none 2 N/A -MAMA G WAVE 6 STIS/FUV ACCUM 52X0.1 G140M 1222 none none none none 2 N/A -MAMA G-32-5 7 STIS/FUV TIME-TA 52X0.1 G140M 1222 none none none none 3 N/A -MAMA G WAVE 8 STIS/FUV ACCUM 52X0.1 G140M 1222 none none none none 3 N/A -MAMA G-32-5 9 STIS/FUV TIME-TA 52X0.1 G140M 1222 none none none none 4 N/A -MAMA G WAVE 10 STIS/FUV ACCUM 52X0.1 G140M 1222 none none none none 4 N/A -MAMA G-32-5 11 STIS/FUV TIME-TA 52X0.1 G140M 1222 none none none none 5 N/A -MAMA G WAVE 12 STIS/FUV ACCUM 52X0.1 G140M 1222 none none none none 5 N/A -MAMA 17600( 8) - 21-Nov-2025 15:10:16 - [ 8] Visit: 03 (GJ 12b visit 3 (repeat of visit 1)) Visit Requirements: Period 12.76142 D AND ZERO-PHASE HJD2459497.183 On Hold Comments: Additional Comments: Third transit of GJ 12b (repeat of visit 1 due to MCE reset). Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 G-32-5 STIS/CCD ACQ F28X50OII MIRROR 1 5 S (5 S) PHASE 0.9876680898 TO I 0.9893006145; SEQ 1-4 NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 2 G-32-5 STIS/CCD ACQ/PEA 0.2X0.05N MIRROR 1 10 S (10 K D S) ------------------------------------------------------------------------------------------------------------------------------------ 3 G-32-5 STIS/FUV TIME-TA 52X0.1 G140M 1222 BUFFER-TIME=835, WAVECAL=NO 1 1350 S -MAMA G (1350 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 WAVE STIS/FUV ACCUM 52X0.1 G140M 1222 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 5 G-32-5 STIS/FUV TIME-TA 52X0.1 G140M 1222 BUFFER-TIME=1357, WAVECAL=NO 1 2600 S SEQ 5-6 NON-INT -MAMA G (2600 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 WAVE STIS/FUV ACCUM 52X0.1 G140M 1222 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 7 G-32-5 STIS/FUV TIME-TA 52X0.1 G140M 1222 BUFFER-TIME=1357, WAVECAL=NO 1 2600 S SEQ 7-8 NON-INT -MAMA G (2600 S) ------------------------------------------------------------------------------------------------------------------------------------ 8 WAVE STIS/FUV ACCUM 52X0.1 G140M 1222 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 9 G-32-5 STIS/FUV TIME-TA 52X0.1 G140M 1222 BUFFER-TIME=1357, WAVECAL=NO 1 2600 S SEQ 9-10 NON-INT -MAMA G (2600 S) ------------------------------------------------------------------------------------------------------------------------------------ 10 WAVE STIS/FUV ACCUM 52X0.1 G140M 1222 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 11 G-32-5 STIS/FUV TIME-TA 52X0.1 G140M 1222 BUFFER-TIME=1357, WAVECAL=NO 1 2600 S SEQ 11-12 NON-INT -MAMA G (2600 S) ------------------------------------------------------------------------------------------------------------------------------------ 12 WAVE STIS/FUV ACCUM 52X0.1 G140M 1222 1 DEF (0 S) -MAMA ------------------------------------------------------------------------------------------------------------------------------------ 17600( 8) - 21-Nov-2025 15:10:16 - [ 9] Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ G-32-5 1 STIS/CCD ACQ F28X50OI MIRROR none none none none 1 N/A II G-32-5 2 STIS/CCD ACQ/PEA 0.2X0.05 MIRROR none none none none 1 N/A K ND G-32-5 3 STIS/FUV TIME-TA 52X0.1 G140M 1222 none none none none 1 N/A -MAMA G WAVE 4 STIS/FUV ACCUM 52X0.1 G140M 1222 none none none none 1 N/A -MAMA G-32-5 5 STIS/FUV TIME-TA 52X0.1 G140M 1222 none none none none 2 N/A -MAMA G WAVE 6 STIS/FUV ACCUM 52X0.1 G140M 1222 none none none none 2 N/A -MAMA G-32-5 7 STIS/FUV TIME-TA 52X0.1 G140M 1222 none none none none 3 N/A -MAMA G WAVE 8 STIS/FUV ACCUM 52X0.1 G140M 1222 none none none none 3 N/A -MAMA G-32-5 9 STIS/FUV TIME-TA 52X0.1 G140M 1222 none none none none 4 N/A -MAMA G WAVE 10 STIS/FUV ACCUM 52X0.1 G140M 1222 none none none none 4 N/A -MAMA G-32-5 11 STIS/FUV TIME-TA 52X0.1 G140M 1222 none none none none 5 N/A -MAMA G WAVE 12 STIS/FUV ACCUM 52X0.1 G140M 1222 none none none none 5 N/A -MAMA 17600( 8) - 21-Nov-2025 15:10:16 - [ 10] Summary Form for Proposal 17600 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 0.2X0.05ND, 52X0.1, F28X50OIII ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, ACQ, ACQ/PEAK, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=1357, BUFFER-TIME=835, WAVECAL=NO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PHASE 0.9876680898 TO 0.9893006145, PHASE 0.9919888386 TO 0.9944376256, Period 12.76142 D AND ZERO-PHASE HJD2459497.183, SEQ 1-4 NON-INT, SEQ 11-12 NON-INT, SEQ 5-6 NON-INT, SEQ 7-8 NON-INT, SEQ 9-10 NON-INT ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140M, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names G-32-5, WAVE ------------------------------------------------------------------------------------------------------------------------------------