Download First light from far-away planet shines the way to new earths

SPACE
First light from far-away planet
shines the way to new earths
09 June 2015
by Jon Cartwright
The star 51 Pegasi is in the northern constellation of Pegasus, also called The Winged Horse. Image ESO/Digitized Sky Survey 2.
It isn’t easy to spot planets far away in our galaxy – normally we can only infer their presence,
from the effect they have on their host stars. But now a group of astronomers has demonstrated
an easier way to study distant worlds, by detecting the first visible light reflected off an
exoplanet.
According to group member and PhD student Jorge Martins at the University of Porto in Portugal,
seeing the exoplanet like this was rather like looking at a streetlamp one hundred metres away, and
being able to glimpse a moth flying right next to it.
‘This detection is equivalent to detecting the light from the lamp, reflected on the wings of the moth,’ he
said.
Since the early 1990s, astronomers have spotted nearly 2 000 planets outside our solar system. Mostly
these exoplanets have revealed themselves by the changes they inflict on their host stars – perhaps a
faint shadow, created as they pass in front of the star; or perhaps a slight wobble, caused by their
gravity pulling at the star as they orbit around it.
Getting any detailed information about an exoplanet is more difficult. Sometimes when an exoplanet
passes in front of a star it is possible to identify colour changes that suggest the presence of certain
chemicals. So far, exoplanets have been found that contain carbon dioxide, methane and even water –
the last one being particularly interesting, since it is thought to be one of the precursors of life.
1
But in order to study an exoplanet, astronomers do not always want to wait for that rare occasion when
it passes in front of its host star. After all, if they want to study a planet in our own solar system, they
simply look at it directly though a telescope, and examine the light reflecting off it from our sun.
In fact, that is pretty much how Martins and his
colleagues have now studied 51 Pegasi b, a
Jupiter-sized exoplanet orbiting the star 51 Pegasi,
which is 50 light years away in the constellation of
Pegasus.
‘Detecting planets in reflected light is a fundamental
goal of exoplanet research,’ said Dr Quinn
Konopacky, a leading astronomer at the University
of Toronto in Canada who was not involved in the
study. ‘This detection of reflected light from a
Jupiter-like planet is a critical first step in reaching
that goal.’
Martins and colleagues’ demonstration was not as
easy as pointing a telescope in the right direction.
51 Pegasi b is about four million times farther away
than our closest planetary neighbour, Venus.
Worse, it reflects only about one ten-thousandth of
the light emitted by its host star.
‘This detection is
equivalent to
detecting the light
from the lamp,
reflected on the
wings of the moth.’
Jorge Martins, University
of Porto in Portugal
Cross correlation function
To pick out the faint signal of 51 Pegasi b, then, the researchers turned to a mathematical function
known as a cross correlation function, which could seek out the spectral colour lines indicative of a
planet and then amplify them. They applied the function to spectral data taken by the European
Southern Observatory’s La Silla Observatory in Chile.
The technique allowed them to view the reflected light spectrum of 51 Pegasi b, and extract a few
parameters from it. These included estimates of the planet’s mass and its reflectivity, or ‘albedo’.
Dr Nicolas Cowan, a leading astronomer at Amherst College in Massachusetts, US, who was not
involved with the study, said that these estimates do not totally agree with those calculated by
independent means, but calls the demonstration ‘exciting’ nonetheless. ‘Not so much because of the
specific detection for this planet … but rather because the technique is promising and this is the first
time that people have used it to look for reflected light,’ he added.
The demonstration of the technique is not the first result to come from the group as part of the
EXOEarths project, which is backed by the EU’s European Research Council. In 2012, for instance, the
group announced the discovery of a planet orbiting in the Alpha Centauri star system that was smaller
than any discovered previously.
The latest technique is expected to reap more substantial rewards when it is used with bigger
telescopes, such as the ESO’s Very Large Telescope or its forthcoming European Extremely Large
Telescope, both in Chile.
‘This method will eventually be used to measure reflected light from many other exoplanets,’ said Dr
Cowan. ‘Exoplanetology is still in its infancy, and these sorts of sketchy first detections quickly give
way to robust results.’
Dr Konopacky said the researchers are ‘pushing current instruments to the limit’.
‘In the future, we will have new instruments that are specifically designed to detect reflected light from
planets more like earth,’ she added.
More info
Jorge Martins
La Silla Observatory
2