Issue #3(25) 2020 Science

The alien hunter’s field manual

The Xtreme Deep Field (XDF), image combines 10 years of Hubble Space Telescope photographs of a narrow patch of sky at the centre of the original Hubble Ultra Deep Field to show some of the most distant galaxies in the observable Universe.
The Xtreme Deep Field (XDF), image combines 10 years of Hubble Space Telescope photographs of a narrow patch of sky at the centre of the original Hubble Ultra Deep Field to show some of the most distant galaxies in the observable Universe.
Joseph Silk Johns Hopkins University, Baltimore, Maryland, USA
McCullen Sandora Elsevier, Pennsylvania, USA

The past few decades have seen remarkable advances in our knowledge of exoplanets (planets orbiting stars outside the solar system). The first, found in the 1990s, were extreme worlds known as hot Jupiters – gas giants with a mass and radius similar to Jupiter, but with much higher surface temperatures because they orbit very close to their host stars. Now, we’ve discovered thousands of exoplanets, many of which are much more like Earth in size and temperature. The latest experiments are now in the process of delivering hundreds of Earth-sized planets.

In the next few decades, we will witness significant advances in not just the detection of exoplanets, but also characterising their properties such as atmosphere, ocean and surface. This has the potential to be even more exciting; since we know life can have a large effect on a planet, this will allow us to determine whether life exists on exoplanets that we can characterise. So far, measurements have been mostly restricted to the atmosphere of about a dozen hot Jupiters. The next few decades will see multiple experiments that, when used in conjunction, will be capable of increasing this number a hundredfold, and allow us to probe Earth-like planets as well.

The goal of many of these upcoming experiments is stated as being “to maximise science return”, or in other words, maximising the total number of Earth-like planets that are characterised. With our current knowledge, having no information about the abundance or characteristics of life throughout the universe, it’s hard to come up with a better goal. However, it pays to draw an analogy with planet detection, because in many ways, the state of planet characterisation today mimics the state of planet detection in the 1990s.

At that time, when detections were limited to extreme hot Jupiters, the goal was to find as many as possible to gain a robust understanding of when, where, and how often planets form. Now that we have detected thousands of exoplanets, we learn less from simply searching for more, and astronomers have shifted focus to observing interesting planetary systems that can be used to test hypotheses about planet formation, occurrence and composition.

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