Exoplanets
The universe abounds with planets around other stars.
The first exoplanet, or planet beyond our solar system, was discovered in the 1990s. Since then, more than 5,000 exoplanets ― an abbreviation of “extrasolar planets” ― have been confirmed around stars in our Milky Way galaxy.
For a long time, scientists thought that other planetary systems were likely to resemble our own. But humanity was in for an eye-opening revelation about what constituted a run-of-the-mill planetary system. The first exoplanet discovered was a “hot Jupiter,” or a Jupiter-like gas giant orbiting astoundingly close to its star ― only 5 million miles (8 million km). That’s closer than Mercury is to our Sun.
The variety of new types of planets that poured in were astounding. In addition to many hot Jupiters, astronomers found:
- Super-Earths: Rocky planets more massive than Earth, but lighter than Neptune
- Hot Neptunes: Neptune-size planets in tight orbits around their stars
- Mini-Neptunes (or sub-Neptunes): Roughly Neptune-size planets thought to have solid inner cores and dense helium-hydrogen atmospheres
- Ultra-hot Jupiters: Jupiter-like gas giant planets orbiting so close to their stars that their temperatures exceed 3,000 degrees Fahrenheit, hot to vaporize most metals
- Super Puffs: Young planets with the density of cotton candy. Their hydrogen/helium atmospheres are so bloated they are nearly the size of Jupiter, but their mass is only several times that of Earth
Hubble has played a critical role in our understanding of exoplanets, both by observing the disks of dust and debris that lead to the formation of planets and by peering into these remote systems to glean information about their worlds’ composition and conditions.
Hubble was the first telescope to resolve protoplanetary disks, or “proplyds,” around stars. These pancake-shaped disks of gas and dust are the starting point for planets. Hubble has also studied changes in proplyds that show how planets form from their dust and debris.
Hubble can only rarely directly view exoplanets ― they are too tiny and too close to their stars. What it does best is study the light from their parent stars that passes through their atmospheres and analyze it for information, a technique called spectroscopy. Astronomers used Hubble to take the first measurements of the atmospheric composition of extrasolar planets.
The question of whether we are alone in the universe is the biggest question astronomy has to offer, and to answer it we must first tackle smaller questions, such as: Can any of the 5,000 planets we’ve found support life? Using spectroscopy, Hubble has identified atmospheres that contain sodium, oxygen, carbon, hydrogen, carbon dioxide, methane, helium, and water vapor. An atmosphere with a combination of oxygen, carbon dioxide and methane would mimic the makeup of our own.
Hubble also helps astronomers confirm the presence of planets around other stars using a technique called gravitational microlensing, in which the gravity of a foreground star briefly bends and amplifies the light of a background star that it lines up with. The exact timing and amount of light amplification can reveal clues to the nature of the foreground star and its accompanying planets.
Hubble helps us realize that planetary systems are varied and diverse ― and don’t follow our early assumption that such systems would resemble our own. As Hubble investigates planets around other stars and reveals their strangeness, it reminds us how extraordinary and unexpected the universe can be.
