Spatially Scanned STIS Spectra of the Exoplanet Host Star 55 Cnc
This paper demonstrates that spatially scanning observations of the exoplanet host star 55 Cnc with the HST/STIS instrument yield high-precision optical/near-IR spectra comparable to standard stare-mode results, successfully characterizing the transit of 55 Cnc e and suggesting a variable increase in its apparent radius between 0.55 and 1.0 m.
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Astronomers have long sought to understand the atmospheres of worlds orbiting distant stars, hoping to find clues about their composition and potential habitability. To do this, they watch as a planet passes in front of its host star, a moment known as a transit. During this event, a tiny fraction of the star's light filters through the planet's atmosphere before reaching our telescopes. By analyzing how the star's brightness changes at different colors of light, scientists can infer what gases or particles might be present in that thin atmospheric layer. However, this task is incredibly difficult because the signal is faint and easily drowned out by the telescope itself. Instruments on space telescopes like the Hubble Space Telescope can introduce their own patterns of noise, such as variations in sensitivity across the detector or interference from the Earth's thermal cycles, which can mimic or hide the subtle signals of an alien atmosphere.
To overcome these hurdles, researchers have developed a technique called spatial scanning. Instead of keeping the telescope pointed at a single spot, which can cause the detector to become overwhelmed by the brightness of a star, the telescope is moved slowly while the observation is taken. This spreads the star's light across many different parts of the detector, preventing any single spot from becoming saturated and allowing for a more even measurement of the light. A team of astronomers recently tested this method using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, focusing on the star 55 Cancri, which hosts a small, rocky planet known as 55 Cancri e. Their goal was to see if this scanning technique could produce cleaner, more reliable data than the traditional method of staring fixedly at the star, and to determine what the transit of this specific planet actually looks like when observed with such precision.
The researchers observed 55 Cancri on multiple occasions, capturing the light of the star as the planet passed in front of it. They compared two different ways of collecting this data: the new spatial scanning method, where the telescope trailed the star across the detector, and the older "stare mode," where the telescope held still and the detector was deliberately overexposed to capture as much light as possible. The team had to process the raw images carefully, removing streaks caused by cosmic rays hitting the detector and correcting for a wavy pattern in the light that appears at longer wavelengths, known as fringing. This fringing is a common nuisance in infrared observations, but the scanning technique, combined with special calibration images, allowed them to smooth it out effectively.
When they analyzed the total amount of light received during the transits, the results were striking. The spatial scanning method proved to be remarkably stable, with the measured brightness of the star varying by only about 30 to 40 parts per million after the researchers removed the known instrumental effects. This level of precision is comparable to the best measurements ever achieved with the Hubble Space Telescope. The data confirmed that the planet 55 Cancri e blocks out about 450 parts per million of the star's light during a transit, a value consistent with previous estimates. More importantly, the scanning method showed fewer systematic errors between different observation sessions than the traditional staring method, suggesting that spreading the light out helps the instrument behave more predictably.
The most intriguing finding, however, emerged when the team looked at the transit depth across different colors of light. They divided the star's spectrum into several narrow bands, ranging from blue to near-infrared, and measured how deep the transit appeared in each band. In all the observations, regardless of whether they used the scanning method or the staring method, the planet appeared larger when viewed in redder light than in bluer light. The apparent size of the planet increased by more than 40 percent as the wavelength stretched from the blue end of the spectrum to the near-infrared. This trend was unexpected because standard atmospheric models for rocky planets often predict the opposite effect or a flat line.
The researchers were careful to note that this increase in apparent size was not an artifact of their new scanning technique, as the same pattern appeared in the older staring-mode data. They also ruled out the possibility that the effect was caused by the fringing pattern they had worked so hard to remove, as the trend persisted even after that correction was applied. While the data is limited and the team cannot yet say for certain what physical process is causing this redward slope, the consistency across different observation strategies suggests it is a real feature of the system. It could be due to clouds or haze in the planet's atmosphere, or perhaps unocculted spots on the star's surface that change the way the light is blocked. The study concludes that spatial scanning with the Hubble Space Telescope is a powerful tool for studying bright stars and their planets, offering a high-quality alternative to traditional methods that may help astronomers better understand the complex and variable nature of exoplanet atmospheres.
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