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Spitzer and Hubble identify atmospheric composition of “sub-Neptune” exoplanet

Built by Lockheed Martin and Ball Aerospace, the Spitzer Space Telescope was launched in 2003 atop a Delta II rocket (7920H). Image Credit: NASA / JPL-Caltech

Built by Lockheed Martin and Ball Aerospace, the Spitzer Space Telescope was launched in 2003 atop a Delta II rocket (7920H). Image Credit: NASA / JPL-Caltech

Scientists have identified the atmospheric composition of an exoplanet larger than Earth but smaller than Neptune by studying it with both the Spitzer and Hubble space telescopes.

Gliese 3470 b, also known as GJ 3470 b, is a 12.6-Earth-mass planet in a close orbit around a red dwarf star. Neptune, in contrast, has more than 17 Earth masses.  Over 80 percent of the exoplanets discovered to date have sizes and masses larger than Earth but smaller than Neptune. They are often described as super-Earths or sub-Neptunes.

Researchers have long been curious as to the chemical composition of these planets, which have no analog in our own solar system. Using both Hubble, which observes in optical light, and Spitzer, which observes in infrared light, they studied the planet’s atmosphere in multiple wavelengths as it transited or passed in front of the star 12 times, and as it was eclipsed by passing behind the star 20 times.

The spectroscopic study, which measured the amount of starlight absorbed during transits as well as the loss of reflected starlight during eclipses, surprised scientists by revealing the planet’s atmosphere to be composed mostly of hydrogen and helium, with almost no heavier elements.

“For the first time, we have the spectroscopic signature of such a world.” noted Bjorn Benneke of the University of Montreal in Canada. “This is a big discovery from the planet-formation perspective. The planet orbits very close to the star, and is far less massive than Jupiter–318 times Earth’s mass–but has managed to accrete the primordial hydrogen/helium atmosphere that is largely ‘unpolluted’ by heavier elements. We don’t have anything like this in the solar system, and that’s what makes it striking.

We expected an atmosphere strongly enriched in heavier elements like oxygen and carbon, which are forming abundant water vapor and methane gas, similar to what we see on Neptune. Instead, we found an atmosphere that is so poor in heavy elements that its composition resembles the hydrogen/helium-rich composition of the Sun,” he emphasized.

Unlike hot Jupiters, which form far from their stars and migrate inward, Gliese 3470 b likely formed in its current location, beginning its life as rock, then rapidly accreting hydrogen gas, though significantly less than a hot Jupiter would, from the protoplanetary disk surrounding the star in its early years.

Spitzer, which launched in 2003 and last year celebrated 15 years in space, has studied a wide variety of exoplanets, including some very distant ones. The telescope has been studying exoplanets’ atmospheres in infrared light since 2005, when it conducted the first ever direct observation of light from an exoplanet, in this case, from two hot Jupiters known as HD 209458 b and TrES-r1.

In 2007, it mapped temperature variations on two other hot Jupiters, HD 189733 b, which experiences extremely high winds, and 149026 b, one of the hottest planets ever found, for which the telescope created a global weather map. That same year, Spitzer identified molecules in the atmospheres of two more hot Jupiters, HD 209458b and HD 189733b, revealing the planets to be drier and more cloudy than scientists thought.

“We had no idea when we designed Spitzer that it would make such a dramatic step in characterizing exoplanets,” Spitzer project scientist Michael Werner of NASA’s Jet Propulsion Laboratory (JPL) said at the time.

Two of Spitzer‘s cameras continued functioning even after the telescope ran out of helium in 2009, marking the transition from its cold phase to its warm phase.

In 2010, in conjunction with the Polish Optical Gravitational Lensing Experiment (OGLE), Spitzer helped find a gas giant 13,000 light years away, using the technique of gravitational microlensing.

Between 2015 and 2017, Spitzer played an important role in helping scientists find the TRAPPIST-1 system, which features seven roughly-Earth-sized planets in tight orbits a round a cool red dwarf star. Three of those planets are located in the star’s habitable zone, where temperatures could allow the existence of surface liquid water.




Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

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