Characterizing Distant Worlds
Closer to home, astronomers have grown confident that there are habitable worlds in our galactic neighborhood as they harvest exoplanets and begin to detect objects close to the Earth in mass. The Hubble Space Telescope wasn’t in on the ground floor of the discovery of planets beyond the Solar System; that was the work of ground-based observers patiently working on small telescopes for decades. However, it has played a central role in going beyond detection to begin to characterize the planets.
With more than 850 exoplanets confirmed, and dozens more being discovered every week, the thrill of discovery is not what it was in the late 1990s. Almost all of these planets have been discovered by an indirect method, where the influence of an orbiting planet is seen in a periodic Doppler shift of the parent star. No one doubts that these planets are real, but the evidence of seeing the planet is more direct. So it was very exiting when Hubble provided the first optical image of an exoplanet in 2008. A planet three times Jupiter’s mass is orbiting the bright star Fomalhaut, twenty-five light-years away.48 The planet is embedded in a dust disk of the young star and was spotted in archival images taken for a different purpose at two different epochs. Another research team has developed special imaging processing techniques that may be able to dig as many as a hundred new exoplanets out of the vast trove of Hubble archival images.
One of the many surprises in exoplanet research was the existence of hot Jupiters, planets like one of the giant planets in our Solar System but orbiting closer to their stars than Mercury does to the Sun. These planets present an opportunity because the chances of an alignment that lead to an eclipse as seen from the Earth greatly increase. Over a hundred exoplanets have been seen to eclipse their parent star, dimming it slightly for a few hours. These events are repeatable and Hubble has observed several exoplanet eclipses with its spectrographs.49 The spectrum of the star shows absorption from a trace amount of heavy elements, but when the transit occurs, some starlight filters through the atmosphere of the gas giant and an extra imprint of absorption is added to the spectrum from ingredients in the planet atmosphere. So far, sodium, oxygen, carbon, and hydrogen were detected in the atmosphere of one planet, and carbon dioxide, methane, and water (or rather, steam!) in the atmosphere of another.50 Both are Jupiter-sized and far too hot to be habitable, but these observations are showing the path for the detection of biomarkers: the chemical imprints of biology in the atmosphere of an exoplanet. This may well be the way we first discover life beyond Earth.
Finally, Hubble has contributed to the statistical understanding of exoplanets. Looking toward the crowded stellar bulge of the Milky Way 26,000 light-years away, the telescope found sixteen exoplanet candidates orbiting a variety of stars.51 Five of the planets are in an extreme category not yet found by any other search method: super-rapid orbiters that whirl around their stars in less than a day. They were discovered in 2006 using the transit method. Extrapolating to the entire galaxy, the survey projects to 6 billion Jupiter-sized planets in the Milky Way. Theory predicts terrestrial planets in roughly equal or greater numbers than giant planets, so the distant worlds are out there, just waiting for us to reel them in.