Editor's Note: Astronomers today officially announced the discovery of the exoplanet 51 Eridani b using the new Gemini Planet Imager instrument. 51 Eridani b is a young, methane-rich gas giant that is a milestone in the ongoing search for Jupiter-like worlds beyond our solar system. The team's findings are published in the journal Science. Scientific American first reported news of the discovery in June, when the researchers disclosed the existence and details of 51 Eridani b during a public talk at a smaller scientific meeting. That story appears below. For more background on the Gemini Planet Imager and breakthrough efforts to take pictures of faraway exoplanets, read Scientific American's feature story, Inside the Race to Glimpse Alien Jupiters.
For the first time, astronomers have taken a picture of a young exoplanet that resembles our solar system’s largest world, Jupiter, in orbit and size. Called 51 Eridani b, the world is the first in a looming wave of discoveries promised by a new generation of planet-hunting instruments, and could help scientists unlock the secrets of how Jupiter and other gas giants form and shape their planetary systems.
Discovered in December 2014 using the newly installed Gemini Planet Imager (GPI) on the Gemini South telescope in Chile, 51 Eridani b may soon be confirmed as the smallest directly-imaged exoplanet to date. The finding was announced yesterday at the “In the Spirit of Lyot” astronomy conference in Montreal, Canada during a presentation by GPI’s principal investigator, the Stanford University astronomer Bruce Macintosh. He and other GPI team members declined to comment for this story. They have submitted a paper detailing their discovery to the journal Science, which has strict rules that bar authors publicizing data before publication.
The new exoplanet orbits the Sun-like star 51 Eridani in the constellation Eridanus, some 96 light-years from Earth. Macintosh and his team estimate that 51 Eridani b is twice as massive as Jupiter, and 2.5 times farther from its star. Unlike our familiar gas giant, however, which is about 4.5 billion years old and a chilly -145 degrees Celsius at its cloud tops, 51 Eridani b is much younger and hotter—no older than 25 million years, with methane-laced clouds heated to nearly 400 degrees C.
Scarcely cooled from its formation, 51 Eridani b is glowing in the infrared like a planet-sized light bulb, which is how the GPI team managed to glimpse it from nearly 100 light-years away. Their image of the planet is essentially a baby picture. Despite being so young and so distant, the giant exoplanet is the closest cousin to Jupiter that astronomers have ever seen.
Over the past two decades, astronomers have discovered thousands of exoplanets through more indirect means, watching as unseen worlds cause the stars they orbit to wobble, dim or brighten. Those detection techniques tend to be most effective for finding exoplanets that are close to their stars with years measured in days or months. Worlds like Jupiter, which has a 12-year orbit around our Sun, would likely emerge only after decades of such indirect scrutiny.
Planet hunters have managed to actually see only a handful of exoplanets, a much more difficult task requiring advanced instrumentation on Earth’s most powerful telescopes. Until 51 Eridani b, all the directly-imaged worlds have been gas giants several times the mass of Jupiter, and many are so large that they stretch the definition of planet and are more akin to shrunken, failed stars. Most lurk at the outer edges of their systems, far from the obscuring glare of their stars.
In comparison to these supersized, far-out, somewhat star-like worlds, Jupiter-like planets are diminutive runts hugging the skirts of their suns. This places them in what could be considered a dead zone for planet detection: too far from their stars to be quickly found by indirect means, and too close for easy direct imaging. That's why even after two decades of explosive exoplanet discoveries, astronomers still only have a vague sense of how common these alien Jupiters are, and by proxy, how often planetary systems like ours pop up elsewhere in the cosmos. Jupiter’s great bulk, it is thought, sculpted our solar system in its infancy, shepherding the formation and orbital evolution of other embryonic worlds and perhaps even seeding the young Earth with water, organic compounds and other necessary ingredients for life. Without Jupiter, there is a good chance neither our planet nor we would even be here.
Bottom-up or top-down?
Consequently, astronomers wish to learn not only how common alien Jupiters are, but also how they arise in the first place. GPI and similar facilities, notably the European Southern Observatory’s new Spectro-Polarimetric High-contrast Exoplanet Research instrument (SPHERE), are custom-built to answer that question. All planets are born in whirling disks of gas and dust surrounding a star, but this environment offers divergent two pathways for giant worlds to form—either from the bottom-up, via the gradual glomming together of ever-larger chunks of material, or from the top-down, via the direct, rapid gravitational collapse of a cold clump of gas.
The rapid collapse of gas to form a top-down world traps a great deal of heat within the newborn giant planet, resulting in an orb that for its first 100 million years should be hotter and brighter than something built from the bottom up. Eventually, the newborn planets cool, and the record of their fiery births is lost. GPI, SPHERE and other next-generation instruments are now surveying hundreds of nearby young stars for the infrared glow of fresh-formed Jupiters, hoping to catch the elusive planets before they cool so they can at last learn how most giant planets come to be.
For now, the secrets of these planetary broods remain concealed. Since the GPI team is mainly using 51 Eridani b’s intrinsic infrared brightness to estimate both its size and its temperature, considerable uncertainty remains about just how big and hot it is. Vexingly, those values seem to fall within a slim gray area that exists between the two gas-giant formation scenarios. Either 51 Eridani b is the smallest world ever directly imaged, glowing hot in the aftermath of its top-down birth, or it is somewhat larger and cooler, not a record-breaker but still the first exoplanet that could be shown to have formed from the bottom-up. Further measurements from GPI, SPHERE and space-based telescopes may provide certainty soon. Either way, the historic outcome will be a major step toward learning just how special our solar system’s largest planet might be.