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| An illustration of the hypothesized cloud of cometary dust that could be producing the glow. (NASA, ESA, Andi James/STScI) |
A new analysis of Hubble data has clinched it: There's too much light in the
space around the Solar System.
Not much extra light, to be sure. Just a subtle, ghostly glow, a faint
excess that can't be accounted for in a census of all the light-emitting
objects.
All the stars and galaxies surrounding the Solar System – and zodiacal
light, aka dust on the Solar System's plane – none of these can explain what
astronomers are now calling "ghost light".
After analyzing 200,000 Hubble images and taking thousands of measurements
in a project called SKYSURF, an international collaboration is sure the
excess light is real.
And, moreover, they can't quite account for it. There are possibilities, but
none of them have been confirmed. Not yet, anyway.
The strongest possibility? A dust component to the Solar System that we
haven't yet directly detected: tiny particles of dust and ice from a
population of comets traveling inwards from the dark reaches of the Solar
System, reflecting sunlight and generating a diffuse, global glow.
This source would be a bit closer to us than the extra light detected by the
New Horizons space probe, which found an optical light excess in the space
beyond Pluto, outside the Solar System.
"If our analysis is correct there's another dust component between us and
the distance where New Horizons made measurements. That means this is some
kind of extra light coming from inside our Solar System,"
says astronomer Tim Carleton
of Arizona State University.
"Because our measurement of residual light is higher than New Horizons, we
think it is a local phenomenon that is not from far outside the Solar
System. It may be a new element to the contents of the Solar System that has
been hypothesized but not quantitatively measured until now."
There's a lot of really bright stuff floating around the Universe: planets,
stars, galaxies, even gas and dust. And generally, the bright stuff is the
stuff we want to look at. So detecting ambient light in the interstitial
places – interplanetary, interstellar, and intergalactic space – is a tricky
thing to do.
However, when we do look, we sometimes find that things aren't as we expect
them to be.
For instance, something that we can't account for in the galactic center is
producing high-energy light. Voyager I found an excess of brightness
associated with hydrogen at the boundary of the Solar System. There's the
New Horizons detection. Things just seem weirdly glowy out there.
The purpose of SKYSURF was to fully characterize the brightness of the sky.
"More than 95 percent of the photons in the images from Hubble's archive
come from distances less than 3 billion miles from Earth. Since Hubble's
very early days, most Hubble users have discarded these sky-photons, as they
are interested in the faint discrete objects in Hubble's images, such as
stars and galaxies,"
says astronomer and Hubble veteran Rogier Windhorst
of Arizona State University.
"But these sky-photons contain important information which can be extracted
thanks to Hubble's unique ability to measure faint brightness levels to high
precision over its three decades of a lifetime."
Across three separate papers, researchers scoured Hubble's archive for the
signs of faint galaxies that we may have missed, and quantified the light
that should be emitted by objects that are known to shine.
The team searching for hidden galaxies determined that not enough galaxies
were missed to account for the extra light.
The resulting excess was, the scientists said, equivalent to a steady glow
emitted by 10 fireflies across the entire sky.
This may not seem like much, but it's enough to know that we're missing
something. And it's important. Increasingly, scientists are finding ways to
see the light between the stars. If there's a local excess, we need to know
about it, since it could skew our understanding of more distant ghostly
glows.
And, of course, there's the impact it could have on our understanding of the
Solar System and how it's put together.
"When we look up at the night sky, we can learn a lot about the Earth's
atmosphere. Hubble is in space,"
says
astronomer Rosalia O'Brien of Arizona State University.
"When we look at that night sky, we can learn much about what is happening
within our galaxy, our Solar System and on big scales as the whole
Universe."
References:
Timothy Carleton et al, SKYSURF: Constraints on Zodiacal Light and
Extragalactic Background Light through Panchromatic HST All-sky
Surface-brightness Measurements: II. First Limits on Diffuse Light at 1.25,
1.4, and 1.6 μm, The Astronomical Journal (2022).
DOI: 10.3847/1538-3881/ac8d02
Rogier A. Windhorst et al, SKYSURF: Constraints on Zodiacal Light and
Extragalactic Background Light through Panchromatic HST All-sky
Surface-brightness Measurements. I. Survey Overview and Methods, The
Astronomical Journal (2022).
DOI: 10.3847/1538-3881/ac82af
Darby M. Kramer et al, SKYSURF-3: Testing Crowded Object Catalogs in the
Hubble eXtreme Deep Field Mosaics to Study Sample Incompleteness from an
Extragalactic Background Light Perspective, The Astrophysical Journal
Letters (2022).
DOI: 10.3847/2041-8213/ac9cca
Rosalia O'Brien et al, SKYSURF-4: Panchromatic Full Sky Surface Brightness
Measurement Methods and Results, arXiv (2022).
DOI: 10.48550/arxiv.2210.08010
Tags:
Space & Astrophysics