Scientists Detect "Wrong-Way" Planet

The Planetary Society
Article By Amir Alexander
August 12, 2009

An international team of scientists has detected the first extrasolar planet orbiting in the "wrong" direction. This means that the planet, designated WASP-17, is circling its star in a direction opposite to the rotation of the star itself. Such a motion, known as a "retrograde orbit," is very unusual since the motions of both star and planet were acquired from the swirling cloud of gas and dust that formed them both. As a result, the planets orbiting the same star almost always move in the same direction, which is the same as the rotation of the star itself.

A retrograde orbit is almost certainly a legacy of a planet's violent past, most likely dating to the planetary system's early days. "Newly formed solar systems can be violent places" explained graduate student David Anderson of Keele University, who is a member of the team that made the discovery. "A near-collision during the early, violent stage of this planetary system could well have caused a gravitational slingshot, flinging WASP-17 into its backwards orbit."

WASP-17 was first detected through the transit photometry technique by the Wide Area Search for Planets (WASP) consortium of British universities, using the WASP-South camera array in South Africa. But in order to detect its retrograde motion the WASP team needed an assist from planet hunters at the Geneva Observatory, who specialize in radial velocity measurements.

According to Darin Ragozzine of the Harvard-Smithsonian Center for Astrophysics astronomers can identify the direction of a planet's orbit because of slight discrepancies in the radial velocity data when a planet transits a star. Because a star is rotating, one side of it is moving towards (or away) from Earth faster than the other side. During a transit, the planet covers first one side of the star and then the other, causing a slight but measurable shift in the radial velocity readings. If during the transit the star first appears to be moving relatively slowly towards the Earth, but then faster as the transit progresses, then the planet is orbiting in the same direction as the star's rotation. But if the reverse is the case – as it is for WASP-17 – then the planet is in a retrograde orbit.

WASP-17 is located about 1000 light years from Earth, and is unusual not only because of the direction of its orbit but also because of its size and low density. Although its mass is only half that of Jupiter, its diameter is nearly twice that of our giant neighbor, which makes WASP-17 the largest known planet. The reason, according to Coel Hellier of Keele University, is related to the planet's unusual orbit. Retrograde motion coupled with a highly eccentric orbit subject the planet to intense tidal forces. Such tidal compression and stretching would have the effect of heating up the planet, causing it to expand to its current bloated dimensions. As a result, Hellier noted, the density of WASP-17 is only one seventieth (1/70) of the density of Earth.

Just as there are moons in retrograde orbits in our solar system, it stands to reason that there are also planets in retrograde orbits, and the discovery of WASP-17 did not therefore come as a complete surprise to planetary scientists. Nevertheless, this highly unusual planet does contribute to our understanding of the birth and life of planets, and adds one more member to the menagerie of strange and wonderful worlds astronomers are uncovering in the depths of space.

(What is?) Gravitational Lensing

This is quite a rare phenomenon but occurs naturally. It happens when two astronomical bodies of extreme mass (like stars, galaxies or free-floating planets) are almost perfectly aligned as we, the observers, see them from our standpoint. What you then observe, when one star passes in front of the other, is not a dimming of the light or an eclipse, but multiple distorted images of the background star appearing in a "ring" like structure around the edge of the gravitational influence of the foreground star!



The reason that happens is because the gravity of the star that is closer to the observer bends the light rays from the further away object, acting as a kind of astrophysical "lens". This phenomenon is called gravitational lensing.



Astronomers take advantage of this rare effect to look for new exo-planets orbiting other stars.

KEPLER sees first light

The centuries-old quest for other worlds like our Earth has been rejuvenated by the intense excitement and popular interest surrounding the discovery of hundreds of planets orbiting other stars. There is now clear evidence for substantial numbers of three types of exoplanets; gas giants, hot-super-Earths in short period orbits, and ice giants. The challenge now is to find terrestrial planets (i.e., those one half to twice the size of the Earth), especially those in the habitable zone→ of their stars where liquid water might exist on the surface of the planet.

The Kepler Mission is specifically designed to survey our region of the Milky Way galaxy to discover hundreds of Earth-size and smaller planets in or near the habitable zone and determine the fraction of the hundreds of billions of stars in our galaxy that might have such planets.


This image zooms into a small portion of Kepler's full field of view -- an expansive, 100-square-degree patch of sky in our Milky Way galaxy. An eight-billion-year-old cluster of stars 13,000 light-years from Earth, called NGC 6791, can be seen in the image. Clusters are families of stars that form together out of the same gas cloud. This particular cluster is called an open cluster, because the stars are loosely bound and have started to spread out from each other.

The area pictured is 0.2 percent of Kepler's full field of view, and shows hundreds of stars in the constellation Lyra. The image has been color-coded so that brighter stars appear white, and fainter stars, red. It is a 60-second exposure, taken on April 8, 2009, one day after the spacecraft's dust cover was jettisoned.

Kepler was designed to hunt for planets like Earth. The mission will spend the next three-and-a-half years staring at the same stars, looking for periodic dips in brightness. Such dips occur when planets cross in front of their stars from our point of view in the galaxy, partially blocking the starlight.

To achieve the level of precision needed to spot planets as small as Earth, Kepler's images are intentionally blurred slightly. This minimizes the number of saturated stars. Saturation, or "blooming," occurs when the brightest stars overload the individual pixels in the detectors, causing the signal to spill out into nearby pixels.

Image credit: NASA/Ames/JPL-Caltech

Folded Space

Ken Wilber on Science and Spirituality

Philosopher Ken Wilber talks about the methods of science and how these can be extended to include investigation of broader phenomena and ideas.

Spirituality and the Three Strands of Deep Science

Introduction to Astronomy - 1 - part 3 (10 min)

Part 3 of the Introduction to Astronomy lecture.



Follow these links for part 1 and part 2.

Introduction to Astronomy - 1 - part 2 (10 min)

Part 2 of the Introduction to Astronomy lecture.

2009-International Year of Astronomy

2009 marks 400 years since the invention of the telescope. IYA2009 is a global effort initiated by the International Astronomical Union and UNESCO to engage and inspire the people of the world by revealing some of the most recent discoveries and developments in astronomy.

You can watch the live video feed from the opening ceremony and opening talks by leading astrophysicists here.

The Planet

The Planet is a Scandinavian production (2006) directed by Johan Söderberg which draws on the cinematographic techniques of nonverbal films such as Godfrey Reggio's Koyaanisqatsi, and Ron Fricke's Baraka. Unlike those films, which are purely visual, The Planet is riddled with a plethora of interviews from leading scientists and environmentalists offering their perspectives on the challenges humanity faces in the 21st century.

New Eyes, New Skies

The next 40 years will see telescopes that far outstrip any ever seen before. Jeff Kanipe profiles two of them. (from Nature Vol 457 Jan 2009)

The armillary and astrolabe are now seldom seen outside museums and antique shops; but the telescope, which joined them in the observatories of early modern Europe 400 years ago, is still at the centre of the astronomical world. In optical precision, in the wavelengths that are used and in their sheer size, they have changed almost beyond recognition.

After two centuries in which they left no records other than the users’ sketches, and a century in which their visions were recordedon photographic plates, they have in the past decades become entirely electronic. And they are now stationed everywhere — oceans, deserts, mountain tops and all kinds of orbit. But the job is still the same: collecting and focusing whatever information the Universe sends our way.

Yet for all its glorious 400-year history, the astronomical telescope’s best days may still be to come. Telescopes currently in development show an unprecedented degree of technical ambition as they seek to provide near-definitive answers to questions that, a generation or two ago, it was hard to even imagine investigating.

To answer these questions, the telescopes profiled here will often work in complementary ways. The infrared capabilities of the James Webb Space Telescope and the radio acuity of the Square Kilometre Array will both be used to probe the Universe at the time of its own ‘first light’ — the birth of the first stars and galaxies. The radio array will map the large-scale structure of the Universe, elucidating the role in that structure of ‘dark matter’ and ‘dark energy’, as will studies of the faintest galaxies by the Large Synoptic Survey Telescope and European Extremely Large Telescope. That behemoth and the orbiting Webb will, in turn, complement each other in their attempts to characterize planets around other stars with unprecedented detail.

This quartet, for all its ambition and expense, does not exhaust the possibilities being explored and wished for. The Atacama Large Millimeter/Submillimeter Array will soon revolutionize astronomy at its chosen wavelengths. Other projects are planned throughout the electromagnetic spectrum and beyond into the new realms of gravitational waves and neutrinos. These instruments are all being designed with specific scientific challenges in mind. But at the same time, all concerned hope devoutly to discover something as strange and unlooked for as Galileo’s mountains on the Moon — or spots on the face of the Sun.

The James Webb Space Telescope

Like the Hubble Space Telescope, to which it is in some ways the successor, the James Webb Space Telescope (JWST) will be the orbital flagship of its generation. But whereas the Hubble sees mainly in the visible and ultraviolet, JWST is optimized for the infrared. That means it can see things hidden from the Hubble and ts like by dust, and peer into the high-redshift epoch just after the Big Bang at objects indiscernible at visible wavelengths — such as the first stars.

Astronomers at the Space Telescope Science Institute in Baltimore, Maryland, started their first plans for a follow-on instrument in 1989 — a year before the Hubble itself was launched. It should finally make it to the launch pad 24 years later. Although its design and cost have changed a few times over the past two decades (see Nature 440, 140–143; 2006), its main mission remains simple and visionary — to study unseen aspects of every phase of the history of the Universe. To do so, the telescope will make use of several innovative technologies, such as ultra-lightweight optical systems made from beryllium, extremely sensitive infrared detectors and a cryocooler that can maintain the mid-infrared detectors at a frosty 7 kelvin indefinitely.

The most striking of the new technologies, though, affects the very heart of the telescope.
JWST’s designers wanted a mirror that would have been too large to fit into the payload fairing of any rocket available. So they designed one in segments, a mirror that could be launched folded up and then deployed to its full 6.5-metre diameter once the telescope settles
into its final orbit, 1.5 million kilometres from Earth. That distance gives the telescope
much more sky to look at than the Hubble gets, and keeps it cooler, too. But it has its downside: as yet there is no way to get there to ix any problems so, unlike, the Hubble, JWST has to work perfectly from the start.

At the moment, says John Mather, Nobel laureate and senior project scientist for JWST, the telescope is designed to last for at least five years, but longer may be possible. It will carry ten years’ worth of fuel, and the presence of the cryocooler means that, unlike earlier infrared missions, its lifetime is not limited by a fixed supply of coolant. “If we are lucky and clever we hope to conserve fuel and perhaps run much longer,” says Mather. “But we can’t promise that.” What Mather thinks he can promise is discovery. “We do not know which came first, black holes or galaxies, and we do not know how it happens that there is a massive black hole at the centre of almost every massive galaxy. If there are any surprises about the early Universe, I am guessing that they will be in these areas.”

JWST is not just about deep space and distant epochs, though; it will also scrutinize the shrouded origins of objects closer to home — such as nascent solar systems, coalescing stars and star clusters amassing within dusty nebulae, says Matt Mountain, director of the Space Telescope Science Institute. But where the telescope will really stand out will be its ability to probe the very early Universe. “JWST is so sensitive,” says Mountain, “that we can take actual spectra of the very earliest objects you can just barely detect with Hubble.”

The Large Synoptic Survey Telescope

Sometimes telescopes see double not because of aberration, but because that is the way the Universe works. The bending of light by intervening masses — called gravitational lensing — means that some galaxies are seen by Earthly observers in more than one place. By adding together survey image after survey image, and so measuring things that no individual image would show, the designers of the Large Synoptic Survey Telescope (LSST) hope to find a significant fraction of the 10,000 or so such images in every square degree of sky. They also hope to open up a neglected dimension in astronomy: time. As well as adding together images of the same part of space taken again and again to reveal new depth, they will compare those images to spot any differences, turning up a wealth of supernovae, asteroids and Kuiper belt objects on the fringe of the Solar System that would otherwise be missed. The telescope’s proponents call it celestial cinematography.

The telescope will suck in celestial data by the terabyte every night, surveying almost
all of the sky visible from Cerro Pachón, Chile, every week. Such coverage is made possible by an 8.4-metre primary mirror, which will be ground so as to provide a field of view of 10 square degrees. That’s 49 times the size of the full Moon, and more than 300 times the field of view of the Gemini telescopes, which have mirrors of similar size optimized for staring in a single spot.

Over ten years, says Željko Ivezič, of the University of Washington in Seattle, the LSST system will look at everything in its field of view about 1,000 times. A massive amount of computing power will be used to correlate, compare and catalogue the torrent of data — and to make them all available on the Internet. Anyone with a computer — students, and amateur and professional astronomers — will be able to participate in the process of scientific discovery. Studies of objects that have been gravitationally lensed should reveal huge amounts about the structure of the Universe in general, and the distribution of dark matter and the effects of dark energy in particular. At the same time, though, LSST will mount a virtual space patrol, looking for potentially hazardous near-Earth asteroids. Astronomers already know where most of the big, killing-off-species-wholesale asteroids are.

LSST will be one of the tools that catalogues the vast majority of lesser asteroids still capable of smashing a city. But with a sensitivity to faint, transient events 1,000 times greater than ever previously achieved, the telescope will not restrict itself to the ‘vermin of the skies’ in Earth’s backyard. It will observe vast distant cataclysms, such as collisions between neutron stars, and is all but sure of discovering whole new categories of transient events.

The project is overseen by the LSST Corporation, comprising more than 100 scientists and two dozen laboratories, universities and institutes based mainly in the United States. Although the project’s design is still being worked out, the main mirror has already been cast. Astronomers with the corporation are hopeful that construction will begin as planned in 2011 and that first light will occur in 2015. In the subsequent ten-year survey, LSST will take stock of every object in the Universe, known, unknown and newly discovered. “For the first time in history,” says Ivezič, “we will catalogue and study more celestial objects than there are people on Earth.”