Hubble's Exciting Universe: Characterizing Planets Around Other Stars
The Hubble Space Telescope has been at the forefront of exploring planets around other stars. Hubble’s sharpness and broad wavelength coverage allowed astronomers for the first time to probe the atmospheres of these worlds, including their chemical composition and weather systems. These groundbreaking techniques server as proof-of-concept for future telescopes to assess extrasolar planets for their potential habitability.
Background
As Hubble was being built, there was no evidence for the existence of planets around other stars. In the absence of any observational evidence, the idea was treated skeptically by some astronomers. However, science fiction stories in movies and television made planets around other stars seem commonplace in such films as Forbidden Planet and the Star Trek television series.
The challenge to astronomers is that the feeble glow of a planet is drowned out by the fierce glow of its parent star. This was first hypothesized by Giordano Bruno in the early 1600s. In the absence of evidence for exoplanets, there was a lingering theory that our solar system was possibly a freak accident.
Hubble was not designed to look for exoplanets. But the first Hubble clue that exoplanets should be common in our galaxy came in 1992 when immense disks of dust—the raw material for planet formation—were photographed swirling around many of the stars in the Orion Nebula, a young star-forming region 1,500 light-years from Earth.
Such planet-assembly disks had been hypothesized by 18th-century philosopher Immanuel Kant, based on the architecture of our solar system. Hubble provided the first direct visible-light proof for such disks. Since then Hubble has helped astronomers catalog a number of disks where planets may be forming. In addition, Hubble has photographed later-generation disks filled with dust from planetary collisions.
Extrasolar Planets Become Reality
Three years after the space telescope’s observations of disks around stars, astronomers identified the first exoplanet orbiting a normal star. The planet, 51 Pegasi b, wasn’t directly observed, but its telltale gravitational tug on its star was measurable. This detection launched a whole new field of research, where telescopes made a census of stars with planets as detected by minute stellar wobbles. The planet 51 Pegasi b is so close to its star that its discovery overturned conventional theories of planet formation and evolution. Apparently, planets could migrate from their birthing ground to winding up precariously close to their star. This was a bonanza for planetary research because distant worlds could be viewed repeatedly as they whirled around their star in a matter of days, not years, as do planets in our solar system.
Astronomers soon found a way to use Hubble’s unique capabilities to view those short-period planets with orbits tilted edge-on so that the planet can been seen passing in front of its star (called a transit). The first transiting exoplanet was discovered in ground-based observations in 2000. Hubble astronomers jumped on this unique kind of observation to provide the first detection of the atmospheric makeup of an exoplanet via spectroscopy in 2001.
Hubble's capabilities allowed astronomers to make detailed measurements of the parent star’s light as it is filtered through a transiting planet’s atmosphere. By studying the starlight, Hubble was used to detected sodium, methane, carbon dioxide, and other elements in the atmospheres of some exoplanets. This technique, called transmission spectroscopy, has been used at least 100 times on other exoplanets.
Besides deducing chemical composition, these observations allow astronomers to assess whether a planetary atmosphere is cloudy, hazy, or clear. Therefore, Hubble has been the first telescope ever to give astronomers a unique view into the atmospheres of planets around other stars. By characterizing the atmospheres of other worlds many light-years away, Hubble ushered in the era of "comparative exoplanetology," where astronomers understand trends in the characteristics of these largely gaseous worlds.
Hubble provided the first probe of planets in a star's habitable zone, where temperatures would allow for liquid water to remain stable for potential oceans. Hubble's visible-light and ultraviolet-light sensitivity to atmospheric conditions will be complementary to the infrared spectroscopy of atmospheric planetary molecules by the upcoming James Webb Space Telescope.
Probing Planetary Atmospheres
Hubble's unique ultraviolet-light sensitivity offers the capability of detecting atmospheres boiling off those planets that are very close to their stars. The evolutionary consequences are that the planets are shrinking. The observations have helped astronomers understand a huge population of exoplanets that are unlike anything found inside our solar system. These unique worlds are larger than Earth but smaller than Neptune. Depending on their composition—rocky or gaseous—they are either "super-Earths" or "mini-Neptunes."
In 2019 Hubble astronomers detected water vapor in the atmosphere of a mini-Neptune called K2-18b, located 110 light-years away. The planet is at the right distance from its star to have a temperate climate where the water doesn't evaporate or freeze. The planet is likely a giant ball of liquid and gas, like Neptune.
Hubble has actually mapped exoplanets in terms of their temperature distribution and water abundance on the day and night side. They are too far away to be photo-mapped. Instead, astronomers use spectroscopy to sample infrared light radiated from a planet. To map an exoplanet’s temperature variations and water abundances, Hubble will observe a planet during a full orbit.
In 2004 Hubble provided one of the first direct images of an exoplanet candidate orbiting the bright star Fomalhaut. Unlike other directly imaged exoplanets, however, nagging puzzles arose early on. The object was unusually bright in visible light, but did not have any detectable infrared heat signature. Astronomers conjecture that the added brightness came from a huge shell or ring of dust encircling the planet that may possibly have been collision-related.
More that 4,000 exoplanets have been discovered so far. But that’s just the tip of the iceberg. Every one of the 100 billion stars in our galaxy should have planets. Astronomers estimate there could be as many as 8 billion inhabitable Earth-like planets. Though these planets are too small for Hubble to see, there’s still plenty for Hubble to do in characterizing alien worlds.