Astrophys.

Research software

Starspot

Controlled simulations of stellar activity and planetary atmospheric signals in exoplanet transmission spectroscopy.

Solar Orbiter view of increased solar activity, with bright coronal structures and dark sunspots.
Image: ESA & NASA/Solar Orbiter/EUI Team

Scientific Question

Can a cool starspot produce a spectral signal that resembles absorption by a planetary atmosphere? Starspot creates controlled experiments in which the physical origin of every signal is known.

TOI-5205b Case Study

Two isolated scenarios are compared under the same conditions: a planet with an H2O atmosphere transiting a homogeneous star, and a constant-radius planet transiting a star with an unocculted 2500 K spot.

Simulation framework

From the stellar disk to the Doppler map

Starspot generates time-resolved synthetic spectra from a defined physical configuration, preserving every input and output needed to reproduce a run.

01

Resolved stellar disk

Cell-by-cell integration with center-to-limb variation, stellar rotation, Rossiter–McLaughlin geometry and configurable active regions.

02

Physical spectra

PHOENIX photosphere and spot models, wavelength-dependent planetary radius, molecular templates and instrumental resolution.

03

Reproducible analysis

VIS/NIR spectral orders, controlled noise regimes, cross-correlation functions and stellar- versus planetary-frame diagnostics.

Spectral comparison

Similar bands do not imply the same origin

At low resolution, the atmospheric and stellar scenarios share broad regions of variation but are not equivalent. After removing their median levels, their spectral shapes have a Pearson correlation of r = 0.269.

Low-resolution transmission spectra for the planetary H2O atmosphere and the 2500 K stellar spot, including their median-centered spectral-shape comparison.
TOI-5205b low-resolution comparison. The upper panel preserves the physical depth levels; the lower panel compares the spectral structure around each median.

Doppler kinematics

Reference frames separate the two signals

The planetary atmosphere follows the orbital velocity and becomes centered near 0 km s−1 in the planetary frame. Starspot contamination remains tied to the star and acquires the opposite trajectory after the same transformation.

NIR cross-correlation maps for the planetary atmosphere in stellar and planetary reference frames under low-noise, CARMENES and E-MARCOT conditions.
Planetary H2O scenario. The Doppler trajectory collapses toward zero velocity in the planetary frame when the signal-to-noise ratio is sufficient.
NIR cross-correlation maps for the 2500 K stellar spot in stellar and planetary reference frames under low-noise, CARMENES and E-MARCOT conditions.
Unocculted 2500 K spot. The contamination is approximately stationary in the stellar frame and follows the opposite path in the planetary frame.

Current results

Discrimination depends on signal quality

The diagnostic below measures peak contrast relative to the CCF background. It is a comparative recovery metric, not a formal statistical significance.

Scenario Channel CARMENES E-MARCOT
Planetary atmosphere VIS 1.30 ± 0.72 3.64 ± 0.93
Planetary atmosphere NIR 2.31 ± 1.08 6.79 ± 1.14
2500 K spot VIS 1.01 ± 0.53 0.98 ± 0.52
2500 K spot NIR 1.28 ± 0.62 3.10 ± 0.78
Low noise

The different Doppler paths are recovered clearly.

CARMENES

One simulated transit does not recover a robust separation.

E-MARCOT

The adopted sensitivity scaling improves recovery, particularly for the planetary NIR signal.

A strong response to an H2O template measures spectral similarity; by itself, it does not establish a planetary origin or an atmospheric detection.

Software Status

Starspot v6.1 is the documented internal research package used for this controlled study. It combines modular Python components, relative paths, bundled scientific inputs, tested workflows and traceable run configurations.

Access

The current code package is available for selected academic review and scientific collaboration. It is not offered as a public download.

Research Context

Starspot and the TOI-5205b case study were developed by Gerardo Mancebo as part of his Master’s thesis in Astronomy and Astrophysics, under the supervision of Dr. Giuseppe Morello.