The young star TW Hydrae is playing "shadow puppets" with scientists observing it with NASA's Hubble Space Telescope.
In 2017, astronomers reported discovering a
shadow sweeping across the face of a vast pancake-shaped gas-and-dust disk
surrounding the red dwarf star. The shadow isn't from a planet, but from an
inner disk slightly inclined relative to the much larger outer disk – causing
it to cast a shadow. One explanation is that an unseen planet's gravity is
pulling dust and gas into the planet's inclined orbit.
Now, a second shadow – playing a game of
peek-a-boo – has emerged in just a few years between observations stored in Hubble's MAST archive. This could be from yet another disk nestled inside
the system. The two disks are likely evidence of a pair of planets under
construction.
This artist's concept is based on Hubble Space Telescope images of gas-and-dust disks around the young star TW Hydrae. Hubble Space Telescope photos show shadows sweeping across the disks encircling the system. The interpretation is these shadows are from slightly inclined inner disks that block starlight from reaching the outer disk, and therefore are casting a shadow. The disks are slightly inclined to each other due to the gravitational pull of unseen planets warping the disk structure. Credits: Artwork: NASA, AURA/STScI for ESA, Leah Hustak (STScI)
TW Hydrae is less
than 10 million years old and resides about 200 light-years away. In its
infancy, our solar system may have resembled the TW Hydrae system, some 4.6
billion years ago. Because the TW Hydrae system is tilted nearly face-on to our
view from Earth, it is an optimum target for getting a bull's-eye-view of a
planetary construction yard.
The second shadow was discovered in
observations obtained on June 6, 2021, as part of a multi-year program designed
to track the shadows in circumstellar disks. John Debes of AURA/STScI for the
European Space Agency at the Space Telescope Science Institute in Baltimore,
Maryland, compared the TW Hydrae disk to Hubble observations made several years
ago.
"We found out that the shadow had
done something completely different," said Debes, who is principal
investigator and lead author of the study published in The
Astrophysical Journal.
"When I first looked at the data, I thought something had gone wrong with
the observation because it wasn't what I was expecting. I was flummoxed at
first, and all my collaborators were like: what is going on? We really had to
scratch our heads and it took us a while to actually figure out an
explanation."
The best solution the team came up with is
that there are two misaligned disks casting shadows. They were so close to each
other in the earlier observation they were missed. Over time they've now
separated and split into two shadows. "We've never really seen this before
on a protoplanetary disk. It makes the system much more complex than we
originally thought," he said.
The simplest explanation is that the
misaligned disks are likely caused by the gravitational pull of two planets in
slightly different orbital planes. Hubble is piecing together a holistic view
of the architecture of the system.
Comparison images from the Hubble
Space Telescope, taken several years apart, have uncovered two eerie shadows
moving counterclockwise across a gas-and-dust disk encircling the young star TW
Hydrae. The disks are tilted face-on to Earth and so give astronomers a
bird's-eye view of what's happening around the star. The left image, taken in
2016, shows just one shadow [A] at the 11:00 o'clock position. This shadow is cast
by an inner disk that is slightly inclined to the outer disk and so blocks
starlight. The picture on the left shows a second shadow that emerged from yet
another nested disk [C] at the 7:00 o’clock position, as photographed in 2021.
The original inner disk is marked [B] in this later view. The shadows rotate
around the star at different rates like the hands on a clock. They are evidence
for two unseen planets that have pulled dust into their orbits. This makes them
slightly inclined to each other. This is a visible-light photo taken with the
Space Telescope Imaging Spectrograph. Artificial color, to enhance details, has
been added.
Credits: NASA, ESA, STScI, and John
Debes (AURA/STScI for ESA); Image Processing: Joseph DePasquale (STScI)
The disks may be proxies for
planets that are lapping each other as they whirl around the star. It's sort of
like spinning two vinyl phonograph records at slightly different speeds.
Sometimes labels will match up but then one gets ahead of the other.
"It does suggest that the two
planets have to be fairly close to each other. If one was moving much faster
than the other, this would have been noticed in earlier observations. It's like
two race cars that are close to each other, but one slowly overtakes and laps
the other," said Debes.
The suspected planets are located
in a region roughly the distance of Jupiter from our Sun. And, the shadows
complete one rotation around the star about every 15 years – the orbital period
that would be expected at that distance from the star.
Also, these two inner disks are
inclined about five to seven degrees relative to the plane of the outer disk.
This is comparable to the range of orbital inclinations inside our solar
system. "This is right in line with typical solar system style architecture,"
said Debes.
The outer disk that the shadows are
falling on may extend as far as several times the radius of our solar system's
Kuiper belt. This larger disk has a curious gap at twice Pluto's average
distance from the Sun. This might be evidence for a third planet in the system.
Any inner planets would be
difficult to detect because their light would be lost in the glare of the star.
Also, dust in the system would dim their reflected light. ESA's Gaia space
observatory may be able to measure a wobble in the star if Jupiter-mass planets
are tugging on it, but this would take years given the long orbital periods.
The TW Hydrae data are from Hubble's Space Telescope Imaging Spectrograph. The James Webb Space Telescope's infrared vision may also be able to show the shadows in more detail.
The Hubble Space Telescope is a
project of international cooperation between NASA and ESA. NASA's Goddard Space
Flight Center in Greenbelt, Maryland, manages the telescope. The Space
Telescope Science Institute (STScI) in Baltimore conducts Hubble science
operations. STScI is operated for NASA by the Association of Universities for
Research in Astronomy, in Washington, D.C.
Source: Hubble Follows Shadow Play Around Planet-Forming Disk | NASA
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