While it’s exciting to hear about the thousands of new exoplanet candidates the Kepler Space Telescope discovers, it can’t give us the one thing we really want: a good look at another world. Kepler finds planets indirectly, by seeking out their shadows as they cross their home stars, but imaging technologies have improved so much in recent years that astronomers have finally been able to directly image exoplanets.
The Gemini Planet Imager, an instrument installed on the Gemini South Telescope in Chile in 2014, uses adaptive optics to sharpen the image of a target star. Being able to sharply resolve the image a star also makes it possible to completely block it out. Blocking the star’s light enables the telescope to see the far dimmer planets, if they’re there. In the last year, GPI has imaged nearly 100 exoplanets this way. As you might guess, the bigger and hotter and farther away from the star the planet is, the easier it is to find, so up until now those exoplanets have been pretty alien to the kinds of planets we’re familiar with in our own solar system.
Recently the Gemini team took the image above, published today in Science, featuring the planet 51 Eridani b on the lower section. The planet is very likely the lowest mass exoplanet ever imaged, at about twice the mass of Jupiter. Until now, the lowest mass exoplanet imaged was the mass of five Jupiters. It’s orbiting its star at a distance of 13 astronomical units (an orbit comparable in this solar system to one between Saturn and Uranus). And its star is quite young--just 20 million years old (our sun is 4.5 billion years old); the star’s young age means that 51 Eridani b has itself only recently, relatively speaking, formed. That’s why, in fact, the GPI was able to detect the planet--a young system is still fairly hot, and still throwing material onto any planets forming in orbit. Accreting material heats the planet, which releases the energy in mostly infrared light. And yet, while being hot enough to image, 51 Eridani b is also the coolest exoplanet imaged; at just 800 degrees Fahrenheit, it’s around 400 degrees cooler than most of the other planets GPI has discovered. Lastly, what makes this exoplanet interesting is its methane-rich atmosphere, making it even more similar to a young Jupiter; most other exoplanets imaged so far have atmospheric carbon in the form of carbon dioxide.
51 Eridani b looks like it will be a good example for astronomers trying to piece together the way our solar system formed. All of its characteristics, especially it’s cooler temperature and distance from its star, mean that it probably formed the same way Jupiter did: by developing a core over a few million years and then pulling gasses down to its atmosphere. Astronomers think that hotter Jovian exoplanets formed much more rapidly, and a solar system developing in this “hot start” way would create only gas planets. A slow-forming “cold start” system like 51 Eridani’s would likely also create rocky Earth-like planets. Astronomers will use the Gemini Planetary Imager to search for more exoplanets like 51 Eridani b that can teach us more about the timeline of our own solar system.