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Planets in Abundance


This Hubble view of the Orion nebula shows several small illuminated objects which are the dust discs of newly-forming stars called proplyds. In the very centre is an elongated on with an even smaller dark one to its right. The insets show scale blow-ups. The dar one shows clearly as a proto-planetary disc on the right (Hubble public gallery).

Planets, Solar Systems and Biogenesis

Hubble confirms abundance of protoplanetary disks around newborn stars

The Hubble Space Telescope has uncovered the strongest evidence yet that the process which may form planets is common in the Milky Way galaxy. Dr. C. Robert O'Dell said observations with the newly repaired telescope clearly reveal that great disks of dust -- the raw material for planet formation -- are swirling around at least half and probably many more of the stars in the Orion Nebula, a region only 1,500 light-years from Earth where new stars are being born. O'Dell and a colleague, Zheng Wen, formerly of Rice and now at the University of Kentucky, surveyed 110 stars and found disks around 56 of them. Because it is easier to detect the star than the disk, it is likely that far more stars are being orbited by protoplanetary material, O'Dell said.

O'Dell first discovered these disks, which he dubbed "proplyds," in Hubble Space Telescope images taken in 1992. HST clearly resolves a young star at the center of each disk. O'Dell has also been able to measure at least a portion of the mass of a dust disk and found that the disk contains enough material to make an earth-like planet.


A Jupiter-sized planet discovered passing in front of a star 100 light years away. Sci Am 2000

The Earth and other planets of the solar system are believed ot hav formed out of just such a disk some 4.5 billion years ago by the coalescing of matter caused by gravitational attraction. O'Dell said the disks in the Orion Nebula presumably contain the same materials that constitute the planets of Earth's solar system -- carbon, silicates, and other base constituents.


Left: Orion nebula. Centre: Distribution of HCN and HCHO in the Orion nebula confirms the existence of organic molecules pivotal to the origins of life. HCN opolymerizes to RNA bases and HCHO to sugars including ribose. Right: Flattened disc of gas and dust about 30 times the diameter of Pluto's orbit is orbiting the infant star HL Tauri shown by radio emission from CO molecules.

The only directly-observed planetary system to date, consists of three earth-sized bodies orbiting a neutron star 1,000 light-years away. Since the neutron star is the burned-out remnant from a stellar explosion, these planets might have formed at the end of the star's life, and so, are not a good indicator of the abundance of planetary systems like our own.

The Hubble images clearly distinguish the central star from the disk and show that stars in Orion that are the mass of our Sun and lower are likely to possess disks. Stars hotter than our Sun might destroy the dusty disks before they can agglomerate into planets, according to O'Dell. Hubble can see the disks because they are illuminated by the hottest stars in the Orion Nebula, and some of them are seen in silhouette against the bright nebula.

One striking HST image shows a dark elliptical disk silhouetted against the bright background of the Orion nebula. "This object represents the most direct evidence uncovered to date for protoplanetary disks," says O'Dell. Hubble's resolution has allowed O'Dell to determine accurately the mass of the outer rim of the disk. It turns out to be at least several times the mass of our Earth. The entire disk is 53 billion miles across, or 7.5 times the diameter of our solar system. The central, reddish star is about one fifth the mass of our Sun.

The disks identified in the Hubble survey are a missing link in the understanding of how planets like those in our planetary system form. Their abundance in a young star cluster shows that the basic material of planets exists around a large fraction of stars. This reinforces the probability that many stars have planetary systems.

Because the Orion star cluster is less than a million years of age, there has not been enough time for planets to agglomerate from the dust within the disks. Many of the stars are still contracting towards the mature status that they will then retain for billions of years. The most massive stars in the cluster have already reached their adult stage of maximum hydrogen fuel burning and their surfaces have become so hot that their radiation heats up the gas left over after star formation. This is visible to observers with binoculars as the Orion nebula, which is in the middle of the region known as the sword of Orion.

Hubble Space Telescope's detailed images confirm more than a century of speculation, conjecture, and theory about the genesis of a solar system. According to current theories, the dust contained within the disks eventually agglomerates to make planets. Hubble's images provide direct evidence that dust surrounding a newborn star has too much spin to be drawn into the collapsing star. Instead, the material spreads out into a broad, flattened disk.

Before the Hubble discovery, remnant dust disks had been confirmed around only four stars: Beta Pictoris, Alpha Lyrae, Alpha Piscis Austrini, and Epsilon Eridani. They are a fraction of the mass of the proplyds in Orion, and might be leftover material from the planet formation process. Less direct detections of circumstellar material around stars in nearby star forming regions have been made by radio and infrared telescopes.

Planets are considered a fundamental prerequisite for the existence of life as we know it. A planet provides a storehouse of chemicals for manufacturing the complex molecules of biology, gravitationally holds an atmosphere of gasses that are used by life, and receives heat and light from the central star to power photosynthesis and other chemical reactions required by life forms.

The gas disc of Beta Pectoris

A team of astronomers using the newly installed Space Telescope Imaging Spectrograph (STIS) instrument on board the Hubble Space Telescope have achieved the most detailed close-up to date of a disk of gas and dust surrounding the young star Beta Pictoris. Analysis of the visible light images reveals new details regarding warps in the disk, supporting the theory that nascent planets may be forming inside and perturbing the disk through their gravitational influence. "Observations of circumstellar disks are very difficult," said Heap. "It's like trying to read street signs when the sun is in your eyes. Most of the light is coming directly from the star. Only a tiny fraction is reflected by material in the surrounding disk. The challenge is to separate the light from the star so we can see the disk. With Beta Pictoris, we need to get in close to the star, because that's where the warps in the disk are. However, the closer we get to the star, the greater its glare is. We resolve this by using a coronagraph in the STIS instrument, which creates an artificial eclipse that blocks the light from the star so we can see the disk."
"Beta Pictoris is like a young Sun. If the Sun can be thought of as being fifty years old, Beta Pictoris is probably not a half year old yet. It's nearly the same type of star, although slightly more massive, and it's right in the neighborhood at a relatively close 60 light years. Its proximity lets us capture such detailed images. It's a perfect place to explore because it may tell us something about our origins," said Heap. "The observed size and shape of the warp doesn't pin down where the planet is or how massive it might be. The planet could be quite close to the star and many times more massive than Jupiter or it may be far out from the star and only ten times the mass of the Earth.


Chart of Recently discovered putative planetary systems
Seven Planet for Seven stars New Scientist 15 June 1996.

51 Pegasi: The First of a Rash of New Planets

Michel Mayor and Didier Queloz (Geneva Observatory) rocked the world with their announcement of a Jupiter-mass object orbiting a nearby Sunlike star. That star's periodic wobbles -- attributed to the gravitational tug of a massive planet -- were quickly confirmed by Geoffrey Marcy (San Francisco State University) and Butler at Lick Observatory (S&T: January 1996, page 38). Marcy and Butler then started analyzing the data they'd long been acquiring on 120 other stars, and out popped two more planets one orbiting 70 virginis and the other 47 Ursae Majoris. These lie at about 0.5 and 2.1 astronomical units from their stars. A new astronomical era was born.


The radial (line-of-sight) velocity of 51 Pegasi varies sinusoidally with a 4.231-day period and 56 m/sec amplitude, suggesting that the star is circled by a companion with a minimum mass of 0.47 Jupiter.

These oscillations are too small for a brown dwarf, which most theorists believe has a lower mass limit of about 20 times that of Jupiter otherwise gravitation cannot contain thermal gas pressures.

David Gray found that the iron absorption line he studied was tilting one way, then the other, in lock step with the putative planet's orbit. His observations were infrequent, and he could only measure the line's shape, not its exact wavelength. But the wiggling he reported in the February 27, 1997, issue of Nature was nearly as large as the motions that betrayed 51 Pegasi's companion. Orbital motions can't change the shape of a spectral line, only its position. "Therefore," Gray wrote in Nature, "the planet hypothesis is no longer an adequate interpretation of the data."

51 Pegasi's brightness has been monitored by Gregory Henry (Tennessee State University) and his colleagues with an automated 30-inch telescope on Arizona's Mount Hopkins. Presumably, this should have revealed some trace of variability if the star is vibrating. But none was found down to an impressive precision of only 2 parts in 10,000 (0.0002 magnitude). The rock-steady sinusoidal nature of the stars' radial-velocity variations also belies stellar fluctuation as the source.

Now planet hunters can breathe a sigh of relief. The star's spectral lines do not in fact change shape but seem to show pure Doppler shifts, Gray and others have concluded. Gray's retraction appears in the January 8th issue of Nature. In the same issue Artie Hatzes and colleagues at McDonald Observatory announce that they too find no sign of spectral line shape changes in 51 Pegasi, using equipment with more than twice the spectral resolution Gray's. Independent studies led by Timothy M. Brown (High Altitude Observatory) and by Hatzes also seem to rule out oddball pulsations in Tau Boötis, whose own putative planet was discovered shortly after 51 Pegasi's.


Giant planets probably form only in cool distant parts of the solar nebula where water condenses into ice, providing abundant material for planetary growth. Planets in hotter regios closer to the forming star accumulate from less common iron and silicon compounds. They end up as smaller rocky bodies like the Earth.
Worlds around other stars Sci. Am. Jan 91.

Several of the newly-discovered planets have large masses and are much closer to their star than Jupiter is to the sun. Alan Burrows has confirmed however that such planets should remain stable to the star's forces as long as they arefurther than 0.04 astronomical units. Mayor thus concluded the Pegasi 'planet' should be stable.

Theorists now believe the outer discs which formed the outer giant planets would also slow these planets down unless they exit the scene soon after their formation. This would lead to their falling into a closer orbit probably knocking out the smaller Earth-like planets as they did so. These suspicions are consistent with the discovery of more heavy elements in the stars with closely orbiting giants, 51 Peg, Rho Cancri and Tau Bootis, suggesting the inner planets may have fallen into the star (or that the initial disc was exceptionally dusty).

There is no clear evidence that any of these planets could support life as we know it. Their very different structures and orbits from those in our solar system suggests we should keep an open mind about the formative laws of planetary systems.


Terrestrial planets Venus to Uranus: It is clear that the extreme variety of our own planetary system and the like variety of the many galilean satellites spells out a very important feature of astronomical law. In four-dimensional space-time gravitational forces are inverse square law. In addition to the natural graduation of temerature noted above in the proposolar disc, prviding for life as we know it on the outer edge of the inner region where water is liquid, the chaotic variation of conditions produced by such non-linear force fields generates a situation of extreme variety in the universe, much like the Mandelbrot set does. While this may seem to make the other planets of our system a little alien, it does guarantee the universe will explore its entire phase space of possibilities virtually guaranteeing it will leave no stone (planet) unturned in its exploration of the possibilities that life will emerge.

References: Scientific American Jan 91, New Scientist 15 Jun 96, Sky Publishing Corp., Hubble Public Pictures.