A science team gains new insight
into the atmosphere of a “mini-Neptune,” a class of planet common in the galaxy
but about which little is known.
NASA’s James Webb Space Telescope
has observed a distant planet outside our solar system – and unlike anything in
it – to reveal what is likely a highly reflective world with a steamy
atmosphere. It’s the closest look yet at the mysterious world, a “mini-Neptune”
that was largely impenetrable to previous observations.
And while the planet, called GJ
1214 b, is too hot to harbor liquid-water oceans, water in vaporized form still
could be a major part of its atmosphere.
“The planet is totally blanketed by
some sort of haze or cloud layer,” said Eliza Kempton, a researcher at the
University of Maryland and lead author of a new paper, published in Nature, on
the planet. “The atmosphere just remained totally hidden from us until this
observation.” She noted that, if indeed water-rich, the planet could have been
a “water world,” with large amounts of watery and icy material at the time of
its formation.
To penetrate such a thick barrier,
the research team took a chance on a novel approach: In addition to making the
standard observation – capturing the host star’s light that has filtered
through the planet’s atmosphere – they tracked GJ 1214 b through nearly its
entire orbit around the star.
The observation demonstrates the
power of Webb’s Mid-Infrared Instrument (MIRI), which views wavelengths of
light outside the part of the electromagnetic spectrum that human eyes can see.
Using MIRI, the research team was able to create a kind of “heat map” of the
planet as it orbited the star. The heat map revealed – just before the planet’s
orbit carried it behind the star, and as it emerged on the other side – both
its day and night sides, unveiling details of the atmosphere’s composition.
“The ability to get a full orbit
was really critical to understand how the planet distributes heat from the day
side to the night side,” Kempton said. “There’s a lot of contrast between day
and night. The night side is colder than the day side.” In fact, the
temperatures shifted from 535 to 326 degrees Fahrenheit (from 279 to 165
degrees Celsius).
Such a big shift is only possible
in an atmosphere made up of heavier molecules, such as water or methane, which
appear similar when observed by MIRI. That means the atmosphere of GJ 1214 b is
not composed mainly of lighter hydrogen molecules, Kempton said, which is a
potentially important clue to the planet’s history and formation – and perhaps
its watery start.
“This is not a primordial
atmosphere,” she said. “It does not reflect the composition of the host star it
formed around. Instead, it either lost a lot of hydrogen, if it started with a
hydrogen-rich atmosphere, or it was formed from heavier elements to begin with
– more icy, water-rich material.”
Cooler Than Expected
And while the planet is hot by
human standards, it is much cooler than expected, Kempton noted. That’s because
its unusually shiny atmosphere, which came as a surprise to the researchers, reflects
a large fraction of the light from its parent star rather than absorbing it and
growing hotter.
The new observations could open the
door to deeper knowledge of a planet type shrouded in uncertainty.
Mini-Neptunes – or sub-Neptunes as they’re called in the paper – are the most
common type of planet in the galaxy, but mysterious to us because they don’t
occur in our solar system. Measurements so far show they are broadly similar
to, say, a downsized version of our own Neptune. Beyond that, little is known.
“For the last almost decade, the
only thing we really knew about this planet was that the atmosphere was cloudy
or hazy,” said Rob Zellem, an exoplanet researcher who works with co-author and
fellow exoplanet researcher Tiffany Kataria at NASA’s Jet Propulsion Laboratory
in Southern California. “This paper has really cool implications for additional
detailed climate interpretations – to look at the detailed physics happening
inside this planet’s atmosphere.”
The new work suggests the planet
might have formed farther from its star, a type known as a red dwarf, then
spiraled gradually inward to its present, close orbit. The planet’s year – one
orbit around the star – takes only 1.6 Earth days.
“The simplest explanation, if you
find a very water-rich planet, is that it formed farther away from the host
star,” Kempton said.
Further observations will be needed
to pin down more details about GJ 1214 b as well as the formation histories of
other planets in the mini-Neptune class. While a watery atmosphere seems likely
for this planet, a significant methane component also is possible. And drawing
broader conclusions about how mini-Neptunes form will require more of them to
be observed in depth.
“By observing a whole population of
objects like this, hopefully we can build up a consistent story,” Kempton said.
More About the Mission
The James Webb Space Telescope is
the world’s premier space science observatory. Webb will solve mysteries in our
solar system, look beyond to distant worlds around other stars, and probe the
mysterious structures and origins of our universe and our place in it. Webb is
an international program led by NASA with its partners, ESA (European Space
Agency), and CSA (Canadian Space Agency).
MIRI was developed through a 50-50
partnership between NASA and ESA. NASA’s Jet Propulsion Laboratory led the U.S.
efforts for MIRI, and a multinational consortium of European astronomical
institutes contributes for ESA. George Rieke with the University of Arizona is
the MIRI science team lead. Gillian Wright is the MIRI European principal
investigator. Alistair Glasse with UK ATC is the MIRI instrument scientist, and
Michael Ressler is the U.S. project scientist at JPL. Laszlo Tamas with UK ATC
manages the European Consortium. The MIRI cryocooler development was led and
managed by JPL, in collaboration with Northrop Grumman in Redondo Beach,
California, and NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Caltech manages JPL for NASA.
For more information about the Webb mission, visit: https://www.nasa.gov/webb
Source: NASA’s
Webb Takes Closest Look Yet at Mysterious Planet | NASA
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