Artist's rendering of the exoplanet 51 Pegasi b, a “hot Jupiter” that closely orbits a star about 50 light-years from Earth. [Image: ESO/M. Kornmesser/Nick Risinger (skysurvey.org)]
The 1995 discovery of the extrasolar planet 51 Pegasi b—a “hot Jupiter” orbiting close to a star in the Pegasus constellation, some 50 light-years away from Earth—marked the first confirmed observation of an exoplanet orbiting a sun-like star. Some 20 years later, 51 Pegasi b now offers grist for another scientific achievement: A team of scientists from Portugal, France, Switzerland and Chile reports that it has successfully obtained a reflected-light spectrum at optical wavelengths from the dim planet (Astro. Astrophys., doi: 10.1051/0004-6361/201425298). The researchers believe that the discovery—the first application of a technique described two years ago—offers a “very valuable proof of concept” that could aid future exoplanet studies.
Detecting reflected visible light from distant exoplanets is devilishly difficult. The planetary signal is orders of magnitude fainter than that of the nearby star—and, because it by definition represents just reflected starlight, the planet’s dim spectrum ends up swamped by the stellar noise.
Efforts to yield up exoplanet spectra have centered around transmission spectroscopy, which measures the spectrum of stellar light filtered through the exoplanet atmosphere during a transit of the planet across the face of the star; and detection of reflected and emission signatures with complex setups and adaptive optics to mask or filter the stellar signal (see “Exoplanets: Getting a Closer Look,” OPN, November 2014). Some of these efforts have been crowned with conspicuous recent success—but only in the infrared band, not in visible light.
To get to a visible spectrum, the team, led by Jorge Martins of the Instituto de Astrofísica e Ciências do Espaço, Portugal, put into practice an approach that Martins and colleagues had outlined in a 2013 paper. The approach began by obtaining very high signal-to-noise measurements of spectra from the star-planet system. The team accomplished this using the HARPS spectrograph instrument of the 3.6-m European Southern Observatory (ESO) telescope at La Silla-Paranal, Chile, at a point in the exoplanet’s orbit when the planet’s day side was fully reflecting the star’s light.
Next, the team used a numerical method that began by mapping the similarity of the observed spectrum against a binary mask—a numerical operator representing the expected spectrum from the stellar type in question—to create a numerical template representing the part of the signal attributable to unreflected starlight. The researchers then applied cross-correlation functions to ratchet up the signal-to-noise ratio of the part of the signal attributable to reflection from the planet, presumed to be similar but not identical to the star’s signal. A number of tests, including adjusting radial-velocity assumptions for the planet's motion and doing runs using simulated data, verified that the signal was real and did not represent random noise.
The data allowed the researchers to infer a number of characteristics of the planet, including its mass (about half that of Jupiter) and orbital inclination (about nine degrees relative to Earth). The team also notes that its method appears to have been successful using the current-generation instruments on the 3.6-m ESO telescope—which suggests that the technique may rack up even better results when applied to next-generation spectrographs on larger scopes like the Very Large Telescope in Paranal.