Author: Dr. Elena K. Müller, ESA Science Mission Directorate, 2026‑09‑10
Abstract
The High‑Energy Observation and Transmission Mission (HOT‑17) is a European Space Agency (ESA) ultraviolet and soft X‑ray observatory launched in 2025. Designed to study exoplanetary atmospheres and stellar high‑energy radiation, HOT‑17 utilizes a 1.5 m anastigmatic telescope equipped with a high‑resolution ultraviolet spectrometer and a soft X‑ray imager. Over a 5‑year nominal mission, the spacecraft has conducted UV and X‑ray transit spectroscopy of 30 transiting exoplanets and monitored stellar flares across a range of spectral types. This technical report summarizes the mission’s scientific objectives, payload design, operational history, performance, and its impact on exoplanetary science.
Introduction
Since the discovery of the first exoplanet in 1992, the field of exoplanetary science has progressed from detection to detailed characterization of planetary atmospheres. Atmospheric escape, driven by stellar high‑energy radiation, shapes the evolutionary pathways of close‑in planets, influencing observable properties such as radius, composition, and habitability. Observations in the ultraviolet (UV) and soft X‑ray regimes provide critical diagnostics of these processes.
HOT‑17 was conceived to address the gap in UV and X‑ray observations of exoplanetary systems. By combining high spectral resolution UV spectroscopy with soft X‑ray imaging, the mission offers a comprehensive view of stellar high‑energy environments and their impact on planetary atmospheres. In addition, HOT‑17 contributes to comparative planetology through observations of the Solar System’s upper atmospheres and magnetospheres.
While this report focuses on the technical aspects and scientific outcomes of the mission, it also highlights the broader contributions to the exoplanet field and future prospects.
Scientific Motivation and Objectives
High‑energy radiation from a host star, especially in the UV and X‑ray bands, can cause significant atmospheric escape from close‑in planets. This escape not only alters planetary mass and radius but also removes atmospheric constituents that are essential for habitability. A quantitative understanding of atmospheric escape rates, composition, and temperature structure requires precise measurements of UV absorption during transit events and the associated high‑energy stellar activity.
The main scientific goals of HOT‑17 are:
- Measure atmospheric escape rates and composition for hot Jupiters, warm Neptunes, and Earth‑like planets in the habitable zone.
- Characterize UV and soft X‑ray emission from host stars to constrain high‑energy radiation environments.
- Investigate correlations between stellar metallicity, age, magnetic activity, and atmospheric evolution of exoplanets.
- Contribute to Earth and Solar System atmospheric studies to enhance comparative planetology.
Mission Overview
HOT‑17 was launched on 15 March 2025 aboard an Atlas‑V 551 vehicle. It entered a Sun‑synchronous elliptical orbit with an aphelion of 1.2 AU and a perhelion of 0.8 AU, allowing long exposure times to bright nearby stars while minimizing solar thermal loading. The spacecraft carries a 1.5 m anastigmatic telescope and two main instruments: a high‑resolution ultraviolet spectrometer (HTUS) and a soft X‑ray imager (SXI). A 5‑year nominal mission lifetime, extendable to 8 years, covers a transit survey of 25 hot Jupiters, 15 warm Neptunes, and 10 Earth‑like planets in the habitable zone of M‑dwarfs.
Technical Design
Optical System
The telescope is a modified Ritchey‑Chrétien design with a 1.5 m primary mirror and a 0.6 m secondary. The optics are constructed from beryllium alloy for low mass and high stiffness. A 2 arcsec field of view ensures stable pointing for bright stars, with a
UV Spectrometer
The HTUS is a cross‑dispersed echelle spectrometer covering 120–300 nm with a spectral resolving power of R > 10,000. It uses a 2048×2048 CCD with deep depletion and back‑illuminated technology for high quantum efficiency (QE > 40%) and low dark current. A 1 mm slit provides a spectral resolution of 0.01 nm, and the instrument can detect absorption features as low as 0.5% of the stellar continuum. The spectrometer’s optical train uses a 4‑mirror configuration, a 30 mm concave grating, and a 10 mm flat field mirror to achieve the required resolution and avoid spectral crowding.
Soft X‑ray Imager
The SXI is a 0.2–2 keV imaging detector consisting of a 2048×2048 CCD. The instrument has a 1 arcsec pixel scale and 0.5 arcsec spatial resolution for bright X‑ray sources. The CCD uses a 4‑stage cooling system to reduce thermal noise to
Payload Architecture
HOT‑17’s payload is designed for high‑resolution spectroscopy, high throughput imaging, and long‑term stability. The telescope uses a lightweight, high‑stiffness structure and a beryllium mirror coating that allows for a 1.5 m aperture in a 30 kg payload. A dedicated thermal control system maintains the mirrors at 260 K ± 0.5 K during operations, while the detectors are thermally isolated and cooled to
Thermal Control System
The spacecraft’s thermal control is based on passive radiators and active heaters. The main telescope structure is maintained at 260 K by a network of heaters and a radiative panel that dissipates heat to deep space. The detectors are cooled by a Stirling cooler to
Attitude Control and Pointing
HOT‑17 employs a 3‑axis reaction wheel system for high‑accuracy pointing (
Operational History
Over the 5‑year nominal mission, HOT‑17 has completed the planned transit survey, with a 90% success rate for scheduled observations. The ultraviolet spectrometer has achieved an average signal‑to‑noise ratio (S/N) of 100 per pixel for stars brighter than V = 8. The soft X‑ray imager has captured high‑energy flares on 20 host stars, with an average of 5 flare events per star per month. The mission also conducted Solar System atmospheric observations, including monitoring the upper atmosphere of Mars and the Io plasma torus in the Jovian system.
Data Products and Accessibility
HOT‑17 provides a suite of data products, including:
- High‑resolution UV transit spectra for 30 exoplanets.
- Soft X‑ray light curves and images for 20 host stars.
- Composite data sets combining UV and X‑ray measurements for 15 planets.
All data are released under the Open Science Initiative, and are available in FITS format via the ESA Science Archive.
Results and Impact
Key scientific findings from the mission include:
- Detection of hydrogen and helium escape from hot Jupiters, with mass‑loss rates up to 10⁻⁸ M⊕/yr.
- Evidence for oxygen and carbon loss from warm Neptunes, implying the removal of heavier molecules.
- Observation of increased UV flux in early‑type M‑dwarfs, providing constraints on the habitability potential of Earth‑like planets in their habitable zones.
- Correlation between stellar metallicity and atmospheric loss rates, suggesting that metal‑rich stars drive stronger atmospheric erosion.
- Discovery of a new mechanism for atmospheric heating driven by high‑energy radiation, based on a comprehensive analysis of the mission’s dataset.
Future Prospects
HOT‑17 has established a robust platform for UV and X‑ray observations of exoplanets, paving the way for future missions that can probe fainter targets and extend the wavelength coverage. The mission’s design and data processing techniques will inform the next generation of exoplanet observatories.
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