Exoplanet LHS 475 b (Transmission Spectrum)

A flat line in a transmission spectrum, like this one, can be exciting – it can tell us a lot about the planet.

Researchers used NASA’s James Webb Space Telescope’s Near-Infrared Spectrograph (NIRSpec) to observe exoplanet LHS 475 b. As this spectrum shows, Webb did not observe a detectable quantity of any element or molecule. Common signatures in a hydrogen-dominated atmosphere, for example, would indicate a light, gaseous atmosphere. Those elements were not detected in LHS 475 b’s spectrum.

The green line represents a pure methane atmosphere, which is not favored since if methane were present, it would be expected to block more starlight at 3.3 microns. The yellow line represents the best-fit model for a featureless spectrum that contains no evidence of the planet’s atmosphere. This model is representative of a planet that has no atmosphere.

The purple line represents a pure carbon dioxide atmosphere and is indistinguishable from a flat line at the current level of precision. An atmosphere made up of pure carbon dioxide is far more difficult to detect, even for Webb’s advanced instruments. “We require very, very precise data to be able to distinguish a pure carbon dioxide atmosphere from no atmosphere at all,” explained Jacob Lustig-Yaeger of the Johns Hopkins University Applied Physics Laboratory. “A pure carbon dioxide atmosphere may be thin like the one on Mars, making it difficult to detect.”

The researchers studying LHS 475 b suggest that an additional, upcoming observation may act as a “tie breaker,” allowing them to identify any presence of carbon dioxide – or any other molecule – or rule everything out and conclude the planet has no atmosphere. Quite simply, additional data are required before a conclusion can be made.

This transmission spectrum of the rocky exoplanet LHS 475 b was captured by Webb’s NIRSpec instrument on August 31, 2022. A transmission spectrum is made by comparing starlight filtered through a planet’s atmosphere as it moves in front of the star to the unfiltered starlight detected when the planet is beside the star. Each of the 56 data points on this graph represents the amount of light that the planet blocks from the star at a different wavelength of light. The data would reveal molecules in the planet’s atmosphere by showing that they increase the apparent size of the planet at only specific wavelengths. No such atmospheric features are observed in this spectrum.

The gray lines extending above and below each data point are error bars that show the uncertainty of each measurement, or the reasonable range of actual possible values. For a single observation, the error on these measurements is extremely small (30 to 50 parts per million).

The observation was made using the NIRSpec bright object time-series mode, which uses a grating to spread out light from a single bright object (like the star LHS 475) and measure the brightness of each wavelength at set intervals of time.

LHS 475 b is a rocky exoplanet approximately that same size as Earth. It orbits a red dwarf star roughly 41 light-years away in the constellation Octans. The planet orbits extremely close to its star and completes one orbit in two Earth-days. The planet’s discovery was confirmed with data from the Webb Telescope.

The background illustration of LHS 475 b and its star is based on current understanding of the planet from Webb spectroscopy. Webb has not captured a direct image of the planet or its atmosphere.

NIRSpec was built for the European Space Agency (ESA) by a consortium of European companies led by Airbus Defence and Space (ADS) with NASA’s Goddard Space Flight Center providing its detector and micro-shutter subsystems.

Credits

Illustration

NASA, ESA, CSA, Leah Hustak (STScI)

Science

Kevin B. Stevenson (APL), Jacob A. Lustig-Yaeger (APL), Erin M. May (APL), Guangwei Fu (JHU), Sarah E. Moran (University of Arizona)