Astronomers Discover Wake of Planet Around Nearby Star

An international team of astronomers today report the discovery of a huge
distorted disk of cold dust surrounding Fomalhaut – one of the brightest
stars in the sky. The most likely cause of the distortion is the
gravitational influence of a Saturn-like planet at a large distance from the
star tugging on the disk. This provides some of the strongest evidence so
far that Solar Systems similar in size, or even bigger than our own, are
likely to exist.

Image: [enlarge] 2. Artist’s impression of the Fomalhaut planetary system. The star is
surrounded by a vast disk of cold dust. The inner planets are sweeping up
the dust creating an inner region devoid of material. Collisions between
comets continually create more material for the disk. A Saturn-like planet,
shown in the foreground, creates a wake through the dust as it orbits the
star. This causes the outer disk to appear lumpy and distorted, as shown on
the left. Illustration by David Hardy.

One hundred planets are already known to exist outside our Solar System –
although none have yet been seen directly by a telescope. These planets have
been nicknamed “hot-Jupiters” since they are roughly the size of the planet
Jupiter, but have orbits much closer to their star than Jupiter is to our
Sun. What is new and exciting about the observation of Fomalhaut is that it
probes the space much more distant from the star – in fact, on scales of the
orbits of Uranus, Neptune and beyond.

“We were amazed to find that the disk is actually bent about the star”, said
team leader Dr. Wayne Holland of the UK Astronomy Technology Centre (UK ATC)
in Edinburgh. “This strongly suggests there is an orbiting giant planet
shaping the dust we see”.

Fomalhaut, which lies in the constellation of Piscis Austrinus (the Southern
Fish) is only about 200 million years old – an infant compared to our own
Sun at 4.5 billion years. This is not the first time that Holland and his
collaborators have observed Fomalhaut.

“Our previous image did not have the resolution to pin-point the detail we
are now seeing” said Dr. Jane Greaves, one of Holland’s collaborators on the
project. “We could see a hole in the disk near the star, and thought it was
cleared out by the gravitational pull of a planet like Jupiter, but we had
no idea there was anything further from the star.”

A “doughnut” with a hole

The disk appears a bit like a doughnut with a hole in the centre around the
star. “That’s one way of describing it”, laughs Greaves. “The Fomalhaut disk
is close to edge-on as we see it, and in the image it looks like the ends of
the ‘doughnut’ are brighter. The disk is about the same size as the Sun’s
Kuiper Belt of comets which orbits outside Neptune and Pluto and so we may
be seeing a region near Fomalhaut that has many millions of comets”.

Suddenly Fomalhaut doesn’t sound like a fun place to be… A big comet that
hit the Earth 65 million years ago is believed to have played a part in the
extinction of the dinosaurs.

The most puzzling feature in the image is that the disk appears to bend
around the star.

“Our models of the Fomalhaut disk suggest that a planet similar in mass to
Saturn is creating a wake or trail of dust”, says team member Dr. Mark
Wyatt. “The gravity of the planet creates points near its orbit called
‘resonances’ where comets get trapped. When two comets collide, they release
a shower of dust that we see as a bright spot in the disk.”

Some years ago, infrared images from NASA’s COBE satellite showed that Earth
is preceded and trailed in its orbit about the Sun by clumps of dust
particles that might have come from ancient comets.

Are Solar Systems common?

“We believe Fomalhaut looks quite similar to our own Solar System when it
was only 200 million years old”, said Ben Zuckerman, Professor of Astronomy
at University of California at Los Angeles. “At that age, a planetary system
would already have formed. We would have trouble seeing it with optical
telescopes because of the shroud of dust.”

Cold-hearted SCUBA

The dust disk is so cold, and emits so little light, that it is completely
invisible to optical telescopes such as the Hubble Space Telescope. Instead
the astronomers used a special telescope and camera which operates in the
so-called “submillimetre” region of the electromagnetic spectrum, which lies
between infrared light and radio waves.

“We used the James Clerk Maxwell Telescope in Hawaii – the world’s largest
telescope dedicated to the study of light at submillimetre wavelengths”,
Holland said. “With a specially cooled camera called SCUBA we were able to
measure the tiny amounts of heat radiated by the dust particles that make up
the disk”.

SCUBA, which was built at the Royal Observatory, Edinburgh, has detectors
that are cooled to just one-tenth of a degree above absolute zero (-273
degrees Celsius). It has already found young nurseries where stars are being
born and has discovered distant galaxies near the edge of the Universe.

What’s next?

So, what’s next for the team? Holland’s team is now analysing SCUBA pictures
of other bright stars, including Vega, which also show evidence of
gravitationally disturbed dusty disks. Indeed the dusty structures visible
in the SCUBA images indicate that massive planets having orbits as large, or
larger, than those of Saturn, Uranus and Neptune must exist around many
other stars. Even though no such extrasolar planet has yet been seen
directly, the SCUBA images yield the first good observational evidence that
massive planets with big orbits are really out there.

There are plans for a new large-scale survey of over 100 nearby stars.
Holland is also leading a team to build a new, more powerful camera
(‘SCUBA-2’).

“SCUBA-2 will be able to image disks, such as the one around Fomalhaut, in
just a few minutes, instead of the many tens of hours it currently takes”,
says Prof. Ian Robson, Deputy Director of the UK ATC. “It will
revolutionise our search for evidence of planetary systems”.

The new observations of Fomalhaut are reported in a paper to be published in
the Astrophysical Journal – the most widely read journal in astronomy. In
addition to the astronomers mentioned in the text, the team includes
astronomers Dr. Iain Coulson (Joint Astronomy Centre, Hawaii), Dr. William
Dent (UK ATC), Prof. Walter Gear (University of Wales, Cardiff), Dr. Chris
McCarthy (Carnegie Institute of Washington) and Dr. Rich Webb (NASA Ames
Research Centre).

IMAGES

[enlarge]
1. SCUBA image of the dust disk around Fomalhaut. This is a false colour
image of the dust emission around Fomalhaut taken with SCUBA at the James
Clerk Maxwell Telescope. The image shows a huge disk-like structure where
the brightest emission, and hence the most dust, is represented by the
brightest colours (yellow in this case). The disk appears flattened because
of the angle from which it is viewed from the Earth. The dust is most likely
composed of silicate grains (sand), although we cannot tell that directly
from these images. The position of the star is indicated by the “star”
symbol. The stretched oval adjacent to the image shows the apparent diameter
of our own planetary system if it were located at the distance of Fomalhaut.
The Fomalhaut image appears like a ‘doughnut’ with a hole in the centre.
It’s possible that this hole has been caused by the accumulation of dust
into planets like Earth. The gravitationally perturbed region is the red
part to the left of the star.

WEBSITES

This article also appears on the UK ATC website at: http://www.roe.ac.uk/atc

Other websites related to this work:

• SCUBA homepage: http://www.jach.hawaii.edu/JACpublic/JCMT/scuba/
• PPARC homepage: http://www.pparc.ac.uk/
• Public Outreach at the Joint Astronomy Centre (Hawaii):
  http://outreach.jach.hawaii.edu/

Fomalhaut

Fomalhaut is the 17th brightest star in the sky and lies in the
constellation of Piscis Austrinus (the Southern Fish). The name “Fomalhaut”
(pronounced “Fo-mal-ought”) derives from the Arabic name for this star, Fum
al Hut meaning “The Fish’s Mouth.” The alternative Latin names for the star,
Os Piscis Meridiani or Os Piscis Notii mean “The Mouth of the Southern
Fish”. The Arabs also called Fomalhaut Difda al Auwel or Al Difdi` al Awwal
(“The First Frog”). Fomalhaut is located about 25 light years away from our
Sun, is estimated to be 2.3 times as massive as the Sun and about 1.7 times
the diameter. The best estimate for the age of the star is 200 million
years.

The UK ATC

The UK Astronomy Technology Centre is located at the Royal Observatory,
Edinburgh (ROE). It is a scientific site belonging to the Particle Physics
and Astronomy Research Council (PPARC). The mission of the UK ATC is to
support the mission and strategic aims of PPARC and to help keep the UK at
the forefront of world astronomy by providing a UK focus for the design,
production and promotion of state of the art astronomical technology.

The ROE

The Royal Observatory, Edinburgh comprises the UK Astronomy Technology
Centre (UK ATC) of the Particle Physics and Astronomy Research Council
(PPARC), the Institute for Astronomy (IfA) of the University of Edinburgh
and the ROE Visitor Centre.

The JCMT

The James Clerk Maxwell Telescope is the world’s largest telescope dedicated
to observations in the submillimetre part of the electromagnetic spectrum.
It is operated by the Joint Astronomy Centre, on behalf of the UK Particle
Physics and Astronomy Research Council, the Netherlands Organisation for
Scientific Research, and the Canadian National Research Council.

PPARC

The Particle Physics and Astronomy Research Council (PPARC) is the UK’s
strategic science investment agency. It funds research, education and public
understanding in four broad areas of science – particle physics, astronomy,
cosmology and space science.

PPARC is government funded and provides research grants and studentships to
scientists in British universities, gives researchers access to world-class
facilities and funds the UK membership of international bodies such as the
European Organisation for Nuclear Research, CERN, the European Southern
Observatory and the European Space Agency. It also contributes money for the
UK telescopes overseas on La Palma, Hawaii, Australia and in Chile, the UK
Astronomy Technology Centre at the Royal Observatory, Edinburgh and the
MERLIN/VLBI National Facility.

This work was also supported, in part, by NSF and NASA grants to UCLA.