MWC 758
[Right] —
Observations taken by the European Southern Observatory's Very Large
Telescope show a protoplanetary disk around the young star MWC 758. The
disk has two spiral arms that extend over 10 billion miles from the
star.
[Left] — A computer model reproduces the
two-spiral-arm structure; the "x" is the location of a putative planet.
The planet, which cannot be seen directly, probably excites the two
spiral arms.
Photo Credit: NASA, ESA,
ESO, M. Benisty et al. (University of Grenoble), R. Dong (Lawrence
Berkeley National Laboratory), and Z. Zhu (Princeton University). Release Images
A team of astronomers is proposing that huge spiral patterns seen
around some newborn stars, merely a few million years old (about one
percent our sun's age), may be evidence for the presence of giant,
unseen planets. This idea not only opens the door to a new method of
planet detection, but also could offer a look into the early formative
years of planet birth.
Though astronomers have cataloged thousands of planets orbiting other
stars, the very earliest stages of planet formation are elusive
because nascent planets are born and embedded inside vast,
pancake-shaped disks of dust and gas encircling newborn stars, known as
circumstellar disks.
The conclusion that planets may betray their presence by modifying
circumstellar disks on large scales is based on detailed computer
modeling of how gas-and-dust disks evolve around newborn stars, which
was conducted by two NASA Hubble Fellows, Ruobing Dong of Lawrence
Berkeley National Laboratory, and Zhaohuan Zhu of Princeton University.
Their research was published in the Aug. 5 edition of The
Astrophysical Journal Letters.
"It's difficult to see suspected planets inside a bright disk
surrounding a young star. Based on this study, we are convinced that
planets can gravitationally excite structures in the disk. So if you
can identify features in a disk and convince yourself those features
are created by an underlying planet that you cannot see, this would be a
smoking gun of forming planets," Dong said.
Identifying large-scale features produced by planets offers another
method of planet detection that is quite different from all other
techniques presently used. This approach can help astronomers find
currently forming planets, and address when, how, and where planets
form.
Gaps and rings seen in other circumstellar disks suggest invisible
planets embedded in the disk. However gaps, presumably swept clean by a
planet's gravity, often do not help show the location of the planet.
Also, because multiple planets together may open a single common gap,
it’s very challenging to estimate their numbers and masses.
Ground-based telescopes have photographed two large-scale spiral arms
around two young stars, SAO 206462 and MWC 758. A few other nearby
stars also show smaller spiral-like features. "How they are created has
been a big mystery until now. Scientists had a hard time explaining
these features," Dong said. If the disks were very massive, they would
have enough self-gravity to become unstable and set up wave-like
patterns. But the disks around SAO 206462 and MWC 758 are probably just
a few percent of the central star's mass and therefore are not
gravitationally unstable.
The team generated computer simulations of the dynamics of a disk and
how a star's radiation propagates through a disk with embedded
planets. This modeling created spiral structures that very closely
resemble observations. The mutual gravitational interaction between the
disk and the planet creates regions where the density of gas and dust
increases, like traffic backing up on a crowded expressway. The
differential rotation of the disk around the star smears these
over-dense regions into spiral waves. "Although it had been speculated
that planets can produce spiral arms, we now think we know how," said
Zhu.
"Simulations also suggest that these spiral arms have rich
information about the unseen planet, revealing not only its position
but also its mass," Zhu said. The simulations show that if there were
no planet present, the disk would look smooth. To make the grand-scale
spiral arms seen in the SAO 206462 and MWC 758 systems, the unseen
planet would have to be bulky, at least 10 times the mass of Jupiter,
the largest planet in our solar system.
The first planet orbiting a normal star was identified in 1995.
Thanks to ground-based telescopes and NASA's Kepler mission, a few
thousand exoplanets have been cataloged to date. But because the
planets are in mature systems, many millions or a few billion years
old, they offer little direct clues as to how they formed.
"There are many theories about how planets form but very little work
based on direct observational evidence confirming these theories," Dong
said. "If you see signs of a planet in a disk right now, it tells you
when, where, and how planets form."
Astronomers will use the upcoming NASA James Webb Space Telescope to
probe circumstellar disks and look for features, as simulated by the
modeling, and will then try to directly observe the predicted planet
causing the density waves.
Contacts
Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4514
villard@stsci.edu
Ruobing Dong
Lawrence Berkeley National Laboratory, and University of California, Berkeley, California
rdong2013@berkeley.edu
Zhaohuan Zhu
Princeton University, Princeton, New Jersey
zhzhu@astro.princeton.edu
Source: HubbleSite
