How Astronomers Pulled a Hidden Planet Out of Webb’s Glare

The James Webb Space Telescope had already recorded the faint light of 51 Eridani b at 4.3 micrometres. Standard processing left the known planet hidden in the glare of its host star. A team led by Rodrigo Ferrer-Chavez applied a new method to the same observation and recovered the planet with a reported signal-to-noise ratio of 6.3. The result, published in Nature on November 4, gives astronomers an extra measurement at a wavelength where carbon dioxide absorbs light.

Webb had seen the same young giant in other infrared bands. NASA’s earlier image of 51 Eridani b includes a filter representing light at 4.1 micrometres and places the system about 97 light-years away. The new result extends that record to 4.3 micrometres, where absorption can make the planet harder to pick out.

What the mask leaves behind

Webb’s near-infrared camera can put a small mask in the light path to block much of a star’s glare. Some starlight still reaches the detector, where it forms a pattern shaped by the telescope’s optics. Standard subtraction estimates that pattern from reference images. A tiny change in the optics between exposures can leave enough of a mismatch to hide a nearby planet. NASA’s coronagraph guide explains the hardware, and Northwestern’s account describes why the residual changes.

Editorial visualization

The new method, called FLIP, models how light travels through Webb’s optics and adjusts that model to match the observation. A small shift of the star relative to the mask, for example, can change the glare pattern. Fitting the model lets the researchers account for that shift before subtracting the simulated starlight. The result is an image in which a faint point can stand out. Caltech’s account of the comparison shows the change on the same exposure.

Editorial visualization

How much difference did it make?

The gains varied with the target and the apparent distance from the star. In the authors’ comparison with the standard KLIP pipeline, an improvement in sensitivity means a fainter source could clear the detection threshold. The largest gain in these examples came from a separate star field, TYC 5899. Nature paper, sensitivity comparison

The authors also saw a much fainter spot in the 51 Eridani image whose origin was inconclusive. The recovered planet could be checked against an expected position because it was already known. In a search for an unknown companion, a similar spot would still need follow-up to establish what caused it. Nature paper, 51 Eridani results

For astronomers studying known planets, the immediate opportunity is to revisit archived images and see whether they contain usable measurements in bands that were too noisy before. A single new brightness measurement can add a constraint to an atmospheric model. The next test for a search is whether a faint candidate turns up again in a separate observation. Caltech’s discussion of the research