About

Debra Fischer is a Professor of Astronomy and of Geology and Geophysics at Yale University. She is associated with the Yale Center for Astronomy and Astrophysics, located in Kline Tower. Her academic role involves research and teaching in the fields of astronomy, geology, and geophysics, contributing to Yale's scientific community.

Research topics

• Physics
• Geology
• Computer Science
• Astronomy
• Business
• Optics
• Advertising
• Astrobiology
• Remote sensing

Selected publications

• Uncovering Hidden Systematics in Extreme-Precision Radial Velocity Measurements

  arXiv (Cornell University) · 2026-01-05

  preprintOpen access

  We identify and correct for small but coherent instrumental drifts in seven years of radial velocity data from the EXtreme PREcision Spectrograph (EXPRES). The systematics are most notable for the six months before and after 2022 January, when EXPRES experienced larger temperature variations, and we see a systematic trough-to-peak amplitude of 2.8 m/s in the radial velocities. This is large enough to mimic or obscure planetary signatures. To isolate and correct these effects, we develop a suite of diagnostics that track two-dimensional échellogram shifts, scalings, and rotation, as well as line bisector spans (LBS) derived from laser frequency comb (LFC) lines. By combining these empirical tracers with instrument telemetry in a multi-dimensional regression, we reduce the EXPRES instrument trend traced with solar RVs from an RMS of 1.32 m/s to 0.43 m/s, a 67% improvement, and the aggregate of twelve chromospherically quiet stars show a 26% reduction in velocity scatter. Our injection-recovery simulations further demonstrate a doubling in sensitivity to low-amplitude planetary signals after correction. When applied to the stellar time series of $ρ$ Coronae Borealis ($ρ$CrB), the correction removes a spurious planet d signal, restoring the integrity of the data. These results highlight the need for long-term monitoring and multi-dimensional calibration diagnostics on the path toward true centimeter-per-second precision in next-generation EPRV instruments.

  PublisherDOI

• Uncovering Hidden Systematics in Extreme-Precision Radial Velocity Measurements

  ArXiv.org · 2026-01-05

  articleOpen access

  We identify and correct for small but coherent instrumental drifts in seven years of radial velocity data from the EXtreme PREcision Spectrograph (EXPRES). The systematics are most notable for the six months before and after 2022 January, when EXPRES experienced larger temperature variations, and we see a systematic trough-to-peak amplitude of 2.8 m/s in the radial velocities. This is large enough to mimic or obscure planetary signatures. To isolate and correct these effects, we develop a suite of diagnostics that track two-dimensional échellogram shifts, scalings, and rotation, as well as line bisector spans (LBS) derived from laser frequency comb (LFC) lines. By combining these empirical tracers with instrument telemetry in a multi-dimensional regression, we reduce the EXPRES instrument trend traced with solar RVs from an RMS of 1.32 m/s to 0.43 m/s, a 67% improvement, and the aggregate of twelve chromospherically quiet stars show a 26% reduction in velocity scatter. Our injection-recovery simulations further demonstrate a doubling in sensitivity to low-amplitude planetary signals after correction. When applied to the stellar time series of $ρ$ Coronae Borealis ($ρ$CrB), the correction removes a spurious planet d signal, restoring the integrity of the data. These results highlight the need for long-term monitoring and multi-dimensional calibration diagnostics on the path toward true centimeter-per-second precision in next-generation EPRV instruments.

  PublisherOA PDF

• Uncovering Hidden Systematics in Extreme-precision Radial Velocity Measurements

  The Astrophysical Journal Supplement Series · 2026-02-01

  articleOpen access

  Abstract We identify and correct for small but coherent instrumental drifts in 7 yr of radial velocity (RV) data from the EXtreme PREcision Spectrograph (EXPRES). The systematics are most notable for the six months before and after 2022 January, when EXPRES experienced larger temperature variations, and we see a systematic trough-to-peak amplitude of 2.8 m s −1 in the radial velocities. This is large enough to mimic or obscure planetary signatures. To isolate and correct these effects, we develop a suite of diagnostics that track two-dimensional échellogram shifts, scalings, and rotation, as well as line bisector spans derived from laser frequency comb lines. By combining these empirical tracers with instrument telemetry in a multidimensional regression, we reduce the EXPRES instrument trend traced with solar RVs from an rms of 1.32–0.43 m s −1 , a 67% improvement, and the aggregate of 12 chromospherically quiet stars shows a 26% reduction in velocity scatter. Our injection/recovery simulations further demonstrate a doubling in sensitivity to low-amplitude planetary signals after correction. When applied to the stellar time series of ρ Coronae Borealis ( ρ CrB), the correction removes a spurious planet d signal, restoring the integrity of the data. These results highlight the need for long-term monitoring and multidimensional calibration diagnostics on the path toward true centimeter-per-second precision in next-generation extreme precision RV instruments.

  PublisherDOI

• The Lowell Observatory Solar Telescope: a fiber feed into the Extreme Precision Spectrometer

  2024-06-14 · 4 citations

  articleOpen access

  The signal induced by a temperate, terrestrial planet orbiting a Sun-like star is an order of magnitude smaller than the host stars’ intrinsic variability. Understanding stellar activity is, therefore, a fundamental obstacle in confirming the smallest exoplanets. We present the Lowell Observatory Solar Telescope (LOST), a solar feed for the EXtreme PREcision Spectrometer (EXPRES) at the 4.3-m Lowell Discovery Telescope (LDT). EXPRES is one of the newest high-resolution spectrographs that accurately measure extreme radial velocity. With LOST/EXPRES, we observe disk-integrated sunlight autonomously throughout the day. In clear conditions, we achieve a <i>R</i> ∼ 137, 500 optical spectrum of the Sun with a signal-to-noise of 500 in ∼ 150s. Data is reduced using the standard EXPRES pipeline with minimal modification to ensure the data are comparable to the observations of other stars with the LDT. During the first three years of operation, we find a daily RMS of 71cm/s. Additionally, having two EPRV spectrometers located in Arizona gives us an unprecedented opportunity to benchmark the performance of these planet-finders. We find a RMS of just 55cm/s when comparing data taken simultaneously with EXPRES and NEID.

  PublisherOA PDFDOI

• The Lowell Observatory Solar Telescope: A fiber feed into the EXtreme PREcision Spectrometer

  arXiv (Cornell University) · 2024-07-10

  preprintOpen access

  The signal induced by a temperate, terrestrial planet orbiting a Sun-like star is an order of magnitude smaller than the host stars' intrinsic variability. Understanding stellar activity is, therefore, a fundamental obstacle in confirming the smallest exoplanets. We present the Lowell Observatory Solar Telescope (LOST), a solar feed for the EXtreme PREcision Spectrometer (EXPRES) at the 4.3-m Lowell Discovery Telescope (LDT). EXPRES is one of the newest high-resolution spectrographs that accurately measure extreme radial velocity. With LOST/EXPRES, we observe disk-integrated sunlight autonomously throughout the day. In clear conditions, we achieve a ~137,500 optical spectrum of the Sun with a signal-to-noise of 500 in ~150s. Data is reduced using the standard EXPRES pipeline with minimal modification to ensure the data are comparable to the observations of other stars with the LDT. During the first three years of operation, we find a daily RMS of 71 cm/s. Additionally, having two EPRV spectrometers located in Arizona gives us an unprecedented opportunity to benchmark the performance of these planet-finders. We find a RMS of just 55 cm/s when comparing data taken simultaneously with EXPRES and NEID.

  PublisherDOI

• The Extreme Stellar-Signals Project III. Combining Solar Data from HARPS, HARPS-N, EXPRES, and NEID

  arXiv (Cornell University) · 2023-09-07 · 1 citations

  preprintOpen access

  We present an analysis of Sun-as-a-star observations from four different high-resolution, stabilized spectrographs -- HARPS, HARPS-N, EXPRES, and NEID. With simultaneous observations of the Sun from four different instruments, we are able to gain insight into the radial velocity precision and accuracy delivered by each of these instruments and isolate instrumental systematics that differ from true astrophysical signals. With solar observations, we can completely characterize the expected Doppler shift contributed by orbiting Solar System bodies and remove them. This results in a data set with measured velocity variations that purely trace flows on the solar surface. Direct comparisons of the radial velocities measured by each instrument show remarkable agreement with residual intra-day scatter of only 15-30 cm/s. This shows that current ultra-stabilized instruments have broken through to a new level of measurement precision that reveals stellar variability with high fidelity and detail. We end by discussing how radial velocities from different instruments can be combined to provide powerful leverage for testing techniques to mitigate stellar signals.

  PublisherOA PDFDOI

• Stellar diameters and temperatures - VI. High angular resolution measurements of the transiting exoplanet host stars HD 189733 and HD 209458 and implications for models of cool dwarfs

  UNC Libraries · 2023-07-21 · 1 citations

  articleOpen access

  We present direct radii measurements of the well-known transiting exoplanet host stars HD 189733 and HD 209458 using the CHARA Array interferometer. We find the limbdarkened angular diameters to be θLD = 0.3848 ± 0.0055 and 0.2254 ± 0.0072 mas for HD 189733 and HD 209458, respectively. HD 189733 and HD 209458 are currently the only two transiting exoplanet systems where detection of the respective planetary companion's orbital motion from high-resolution spectroscopy has revealed absolute masses for both star and planet. We use our new measurements together with the orbital information from radial velocity and photometric time series data, Hipparcos distances, and newly measured bolometric fluxes to determine the stellar effective temperatures (Teff = 4875 ± 43, 6092 ± 103 K), stellar linear radii (R* =0.805±0.016, 1.203±0.061 R), mean stellar densities (ρ* =1.62±0.11, 0.58 ± 0.14 ρ), planetary radii (Rp = 1.216 ± 0.024, 1.451 ± 0.074 RJup), and mean planetary densities (ρp = 0.605 ± 0.029, 0.196 ± 0.033 ρJup) for HD 189733b and HD 209458b, respectively. The stellar parameters for HD 209458, an F9 dwarf, are consistent with indirect estimates derived from spectroscopic and evolutionary modelling. However, we find that models are unable to reproduce the observational results for the K2 dwarf, HD 189733. We show that, for stellar evolutionary models to match the observed stellar properties of HD 189733, adjustments lowering the solar-calibrated mixing-length parameter to αMLT =1.34 need to be employed.

  PublisherDOI

• EXPRES IV: Two Additional Planets Orbiting $ρ$ Coronae Borealis Reveal Uncommon System Architecture

  arXiv (Cornell University) · 2023-06-12

  preprintOpen access

  Thousands of exoplanet detections have been made over the last twenty-five years using Doppler observations, transit photometry, direct imaging, and astrometry. Each of these methods is sensitive to different ranges of orbital separations and planetary radii (or masses). This makes it difficult to fully characterize exoplanet architectures and to place our solar system in context with the wealth of discoveries that have been made. Here, we use the EXtreme PREcision Spectrograph (EXPRES) to reveal planets in previously undetectable regions of the mass-period parameter space for the star $ρ$ Coronae Borealis. We add two new planets to the previously known system with one hot Jupiter in a 39-day orbit and a warm super-Neptune in a 102-day orbit. The new detections include a temperate Neptune planet ($M{\sin{i}} \sim 20$ M$_\oplus$) in a 281.4-day orbit and a hot super-Earth ($M{\sin{i}} = 3.7$ M$_\oplus$) in a 12.95-day orbit. This result shows that details of planetary system architectures have been hiding just below our previous detection limits; this signals an exciting era for the next generation of extreme precision spectrographs.

  PublisherOA PDFDOI

• An understanding of the shoulder of giants: Jovian planets around late k dwarf stars and the trend with stellar mass

  UNC Libraries · 2023-07-21 · 1 citations

  articleOpen access

  Analyses of exoplanet statistics suggest a trend of giant planet occurrence with host star mass, a clue to how planets like Jupiter form. One missing piece of the puzzle is the occurrence around late K dwarf stars (masses of 0.5-0.75 M ⊙ and effective temperatures of 3900-4800 K). We analyzed four years of Doppler radial velocity (RVs) data for 110 late K dwarfs, one of which hosts two previously reported giant planets. We estimate that 4.0% ± 2.3% of these stars have Saturn-mass or larger planets with orbital periods &lt;245 days, depending on the planet mass distribution and RV variability of stars without giant planets. We also estimate that 0.7% ± 0.5% of similar stars observed by Kepler have giant planets. This Kepler rate is significantly (99% confidence) lower than that derived from our Doppler survey, but the difference vanishes if only the single Doppler system (HIP 57274) with completely resolved orbits is considered. The difference could also be explained by the exclusion of close binaries (without giant planets) from the Doppler but not Kepler surveys, the effect of long-period companions and stellar noise on the Doppler data, or an intrinsic difference between the two populations. Our estimates for late K dwarfs bridge those for solar-type stars and M dwarfs, and support a positive trend with stellar mass. Small sample size precludes statements about finer structure, e.g., a "shoulder" in the distribution of giant planets with stellar mass. Future surveys such as the Next Generation Transit Survey and the Transiting Exoplanet Satellite Survey will ameliorate this deficiency.

  PublisherDOI

• EXPRES. IV. Two Additional Planets Orbiting ρ Coronae Borealis Reveal Uncommon System Architecture

  The Astronomical Journal · 2023-07-05 · 14 citations

  articleOpen access

  Abstract Thousands of exoplanet detections have been made over the last 25 years using Doppler observations, transit photometry, direct imaging, and astrometry. Each of these methods is sensitive to different ranges of orbital separations and planetary radii (or masses). This makes it difficult to fully characterize exoplanet architectures and to place our solar system in context with the wealth of discoveries that have been made. Here, we use the EXtreme PREcision Spectrograph to reveal planets in previously undetectable regions of the mass–period parameter space for the star ρ Coronae Borealis. We add two new planets to the previously known system with one hot Jupiter in a 39 day orbit and a warm super-Neptune in a 102 day orbit. The new detections include a temperate Neptune planet ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>M</mml:mi> <mml:mi>sin</mml:mi> <mml:mi>i</mml:mi> <mml:mo>∼</mml:mo> <mml:mn>20</mml:mn> </mml:math> M ⊕ ) in a 281.4 day orbit and a hot super-Earth ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>M</mml:mi> <mml:mi>sin</mml:mi> <mml:mi>i</mml:mi> <mml:mo>=</mml:mo> <mml:mn>3.7</mml:mn> </mml:math> M ⊕ ) in a 12.95 day orbit. This result shows that details of planetary system architectures have been hiding just below our previous detection limits; this signals an exciting era for the next generation of extreme precision spectrographs.

  PublisherOA PDFDOI

Recent grants

• The Planet Whisperer: Toward Characterizing Low-mass Planets from Doppler Surveys in the Presence of Stellar Activity

  NSF · $550k · 2016–2020

• Optimal Coupling of Light to Spectrographs for Precision Radial Velocities

  NSF · $361k · 2012–2014

• Tunable Fabry-Perot Frequency Comb for Precision Doppler Measurements

  NSF · $735k · 2015–2017

• MRI: Development of Chiron: CTIO High Resolution Spectrometer

  NSF · $604k · 2009–2011

• Searching for Rocky Planets with APF

  NSF · $390k · 2009–2011

Frequent coauthors

• Geoffrey W. Marcy

  Health Awareness (United States)

  303 shared

• R. Paul Butler

  Carnegie Institution for Science

  181 shared

• Andrew W. Howard

  California Institute of Technology

  143 shared

• John Asher Johnson

  132 shared

• Howard Isaacson

  104 shared

• Steven S. Vogt

  102 shared

• Jason T. Wright

  Pennsylvania State University

  99 shared

• Gregory W. Henry

  96 shared

Education

• Ph.D., Astronomy & Astrophysics

  University of California Santa Cruz

  1998

• M.S., Physics & Astronomy

  San Francisco State University

  1991