This artist's illustration shows the theoretical
internal structure of the exoplanet GJ 3470 b. It is unlike any planet
found in the Solar System. Weighing in at 12.6 Earth masses the planet
is more massive than Earth but less massive than Neptune. Unlike
Neptune, which is 3 billion miles from the Sun, GJ 3470 b may have
formed very close to its red dwarf star as a dry, rocky object. It then
gravitationally pulled in hydrogen and helium gas from a circumstellar
disk to build up a thick atmosphere. The disk dissipated many billions
of years ago, and the planet stopped growing. The bottom illustration
shows the disk as the system may have looked long ago. Observation by
NASA's Hubble and Spitzer space telescopes have chemically analyzed the
composition of GJ 3470 b's very clear and deep atmosphere, yielding
clues to the planet's origin. Many planets of this mass exist in our
galaxy. Credits: Artist's Illustration: NASA, ESA, and L. Hustak (STScI); Science: NASA, ESA, and B. Benneke (University of Montreal). Hi-res image
Two NASA space telescopes have teamed up to identify, for the first
time, the detailed chemical "fingerprint" of a planet between the sizes
of Earth and Neptune. No planets like this can be found in our own solar
system, but they are common around other stars.
The planet, Gliese 3470 b (also known as GJ 3470 b), may be a cross
between Earth and Neptune, with a large rocky core buried under a deep
crushing hydrogen and helium atmosphere. Weighing in at 12.6 Earth
masses, the planet is more massive than Earth, but less massive than
Neptune (which is more than 17 Earth masses).
Many similar worlds have been discovered by NASA's Kepler space
observatory, whose mission ended in 2018. In fact, 80% of the planets in
our galaxy may fall into this mass range. However, astronomers have
never been able to understand the chemical nature of such a planet until
now, researchers say.
By inventorying the contents of GJ 3470 b's atmosphere, astronomers
are able to uncover clues about the planet's nature and origin.
"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," said Björn Benneke of the University of Montreal, Canada. "We don't have anything like this in the solar system, and that's what makes it striking."
"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," said Björn Benneke of the University of Montreal, Canada. "We don't have anything like this in the solar system, and that's what makes it striking."
Astronomers enlisted the combined multi-wavelength capabilities
NASA's Hubble snd Spitzer space telescopes to do a first-of-a-kind study
of GJ 3470 b's atmosphere.
This was accomplished by measuring the absorption of starlight as the
planet passed in front of its star (transit) and the loss of reflected
light from the planet as it passed behind the star (eclipse). All
totaled, the space telescopes observed 12 transits and 20 eclipses. The
science of analyzing chemical fingerprints based on light is called
"spectroscopy."
"For the first time we have a spectroscopic signature of such a
world," said Benneke. But he is at a loss for classification: Should it
be called a "super-Earth" or "sub-Neptune?" Or perhaps something else?
Fortuitously, the atmosphere of GJ 3470 b turned out to be mostly
clear, with only thin hazes, enabling the scientists to probe deep into
the atmosphere.
"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", said Benneke. "Instead, we
found an atmosphere that is so poor in heavy elements that its
composition resembles the hydrogen/helium rich composition of the Sun."
Other exoplanets called "hot Jupiters" are thought to form far from
their stars, and over time migrate much closer. But this planet seems to
have formed just where it is today, says Benneke.
The most plausible explanation, according to Benneke, is that GJ 3470
b was born precariously close to its red dwarf star, which is about
half the mass of our Sun. He hypothesizes that essentially it started
out as a dry rock, and rapidly accreted hydrogen from a primordial disk
of gas when its star was very young. The disk is called a
"protoplanetary disk."
"We're seeing an object that was able to accrete hydrogen from the
protoplanetary disk, but didn’t runaway to become a hot Jupiter," said
Benneke. "This is an intriguing regime."
One explanation is that the disk dissipated before the planet could
bulk up further. "The planet got stuck being a sub-Neptune," said
Benneke.
NASA's upcoming James Webb Space Telescope will be able to probe even
deeper into GJ 3470 b's atmosphere thanks to the Webb's unprecedented
sensitivity in the infrared. The new results have already spawned large
interest by American and Canadian teams developing the instruments on
Webb. They will observe the transits and eclipses of GJ 3470 b at light
wavelengths where the atmospheric hazes become increasingly transparent.
The Hubble Space Telescope is a project of international cooperation
between NASA and ESA (European Space Agency). NASA's Goddard Space
Flight Center in Greenbelt, Maryland, manages the telescope. The Space
Telescope Science Institute (STScI) in Baltimore, Maryland, conducts
Hubble science operations. STScI is operated for NASA by the Association
of Universities for Research in Astronomy in Washington, D.C.
The Jet Propulsion Laboratory in Pasadena, California, manages the
Spitzer Space Telescope mission for NASA's Science Mission Directorate
in Washington, D.C. Science operations are conducted at the Spitzer
Science Center at Caltech in Pasadena. Space operations are based at
Lockheed Martin Space Systems in Littleton, Colorado. Data are archived
at the Infrared Science Archive housed at IPAC at Caltech. Caltech
manages JPL for NASA.
Editor: Lynn Jenner
Source: NASA/Solar System and Beyond
