First Extrasolar Reflected Starlight Detected

ESO/M. Kornmesser/Nick Risinger (skysurvey.org) Artist's impression of 51 Pegasi, a star slightly brighter than the sun and its companion.

Astronomers have found a way to split the light from another star so that they can detect the component reflected off an orbiting planet. The technique could become a powerful new tool for investigating the growing database of extrasolar planets in more detail.

Twenty years ago this October, the discovery of 51 Pegasi b launched a new era in astronomy. The first planet detected around a star other than the sun (aside from two objects orbiting a pulsar), 51 Pegasi b proved our solar system was not unique.

Rather than orbiting stars, planets engage in a kind of dance, like the one New Horizons revealed for Pluto and Charon. The Doppler effect alters the light from the star slightly in the course of this dance, stretching wavelengths when the star retreats and squeezing as it returns.

The closeness of 51 Pegasi b to its parent star creates a powerful gravitational effect and a short orbital period that has proved easy to track. This also means it receives, and reflects, plenty of light. Unfortunately, this closeness makes it particularly hard for us to detect that light, since anything reflected is lost in the parent star's glare.

However, Jorge Martins of the Universidade do Porto, Portugal, realized we don't need to see a planet's light on its own to detect its influence. Even large planets are much lighter than stars, so they travel much greater distances in the orbital dance. This in turn means they move a lot faster than the star, so their light gets shifted considerably further to the red end of the spectrum when moving away and to the blue end when coming towards us. These shifts are also in the opposite direction to that of the parent star at any particular time.

Martins reasoned that if we take the light we receive from Pegasi b and searched for signs of a small component that is shifted further, and in the opposite direction, to most of what we detect, this would represent the planet's contribution. Returning to where it all began, he used the La Silla Paranal Observatory to examine the light from 51 Pegasi, and was rewarded with a distinct spectrum.

Not only did the light that Martins detect confirm the technique's validity, it also revealed the exact inclination of 51 Pegasi b's orbit relative to the Earth (9°), which we had previously only been able to estimate. In Astronomy and Astrophysics, Martins also reports that 51 Pegasi b's mass is 0.4-0.52 times that of Jupiter – which is half the estimates made at the time of its discovery.

Previously, we had no idea of 51 Pegasi b's size, but the brightness suggests it must have a radius almost twice Jupiter's and reflect a higher proportion of the light it receives.

The arrival of new, more powerful telescopes this decade should allow astronomers to apply this technique to many planets more distant from Earth or their parent star.

H/T: Science Magazine

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