ARE WE ALONE: THE EXOPLANET FACTOR
OTHER SOLAR SYSTEMS
Planets also exist that orbit other stars, in our and other galaxies. This has long been suspected, but because the nearest stars are so many light years away, any such planet would appear from Earth to be so close to its parent star that until recently it would be extremely difficult to detect. The discoveries of Smiley and Karla in our Solar System lent new evidence, soon confirmed, that other stars may have solar systems like our own, since many were known to have cometary belts of debris. Astronomers now believe extrasolar giant planets, known as exoplanets, and brown dwarfs (giant planets having slightly too little mass to become stars) probably orbit more than half, perhaps most, of all the 100 billion stars in the Milky Way. This phenomenally changes the odds with regard to the possibilities of life elsewhere in the universe.
Observations of the Orion Nebula, in which new stars are forming, made by the Hubble Space telescope, suggested huge numbers of stars might have planets in orbit around them. Fifteen of these stars have discs of swirling dust around them - raw materials of new worlds, according to Dr Robert O’Dell of Rice University, Houston TX. Dust surrounding a star spins too fast to fall into it. The star becomes heated by nuclear fusion and the dust forms itself into great rings, like those surrounding Saturn. These break up and the dust grains form into larger "globules". When one reaches a diameter of 200 miles or more, its gravity forces it to become spherical, like Earth.
The first unconfirmed discovery in 1991, by Jodrell Bank, of a planet about the size of Earth found orbiting another star, was subsequently found to be due to a miscalculation in the computer program. However, the next discovery of two other planets beyond our own Solar System, 2000 light years away and with 2 month and 3 month orbits, was announced in October 1991 by Alex Woszczan, a Polish émigré doctor working with Dale Frail at Arecibo Observatory in Puerto Rico. When eventually confirmed, it was thought to be the most important discovery in planetary science since Copernicus had put the Sun at the centre of the solar system.
These planets, which would be lifeless, baked by radiation from their pulsar parent, were said to be one and a half times the size of Earth, orbiting a millisecond pulsar that would once have been accompanied by another star. This companion star would eventually have been destroyed by the gravitational pull of the pulsar, leaving a disc of material orbiting the pulsar in which planets such as these might form.
Since August 1988 there had also been inferred to be a planet one and a half times the mass of Jupiter orbiting the bright star 36 Ursae Majoris A, with a three-year orbital period.
In 1992 Geoff Marcy and Paul Butler of San Francisco State Planet Search, using Lick Observatory outside Son Jose CA, claimed a planet orbiting 47 Ursae Majoris. It was three times the size of Jupiter and an equivalent distance as Mars and Jupiter in our solar system. This was the first extrasolar planet discovered to partially resemble Jupiter. It was in a low-eccentricity orbit around its host G0V-type star, which was not unlike our Sun. The planet had a radius about double that of Earth, and had a period of 1102.86 days. The press release said it "probably has a region in its atmosphere where the temperature would allow liquid water."
They also found a planet orbiting 70 Virginis in the same year. This last planet, 70 B Virginis, was said to be eight times larger than Jupiter, and was thought to have a cloud top temperature of circa 85°C, with a considerable possibility of a large number of moons of its own. Its orbit had a high eccentricity of 0.4. It was therefore the prototype of the "eccentric"-type planets, and appeared to be either a low-mass brown dwarf or a very massive Jovian planet.
There has also been evidence to suggest new stars similar to the Sun in the Taurus/Aurigae complex, with nine larger planets, or stars like Jupiter, in orbit around them. In 1994, Prof Alexander Wolsczczan of Pennsylvania State University, using the Arecibo radio telescope in Puerto Rico, via the pulsar-timing method, claimed the existence of three Earth-sized planets in another Solar System. Two terrestrial-mass planets (A, B) had been found in 1991, and a third moon-sized body (C) had been detected that year. The star they orbit, PSR B1257+12, a pulsar star emitting deadly X0-rays and gamma rays, is 1,200 light years from Earth in the constellation Virgo and just 10 miles in diameter. The planets posed a mystery for astrophysicists, as no planet ought to form in such an environment. If in our Solar System, 2 of its 3 planets would be within the orbit of Mercury as they orbit at distances of 20, 35 and 44 million miles respectively. Their years would equal 25.3, 66.5 and 98.2 Earth days. It was suggested they had accreted and formed after the supernova.
In October 1995 the star 51 Pegasi, 51 light years away, was claimed by 2 Swiss astronomers, Michel Mayor and Didier Queloz from the University of Geneva, to have a planet 47% the size of Jupiter orbiting it every 4.2 days. It was detected because of "wobbles" in the image of the star found using Doppler spectroscopy. The discovery had been confirmed on 3 July 1995 when the wobble caused a slight colour shift, as they had predicted. This planet, 51 B Pegasi, was the first of the confirmed Jovian planets discovered around a nearby solar-type star, and could theoretically support life as it lies near enough to its parent star to be rocky. It should be able to retain an atmosphere, although its temperature must exceed a subsolar 1200°C. This was the discovery that made astronomers and scientists sit up and take note. Previous finds had been around unsuitable host stars such as pulsars, which other astronomers had regarded with little interest, because they had no expectations of the possibilities of life there. This discovery broke astronomers’ expectations of where Jovians can exist.
In 1996, after several more discoveries, a red dwarf star, Lalande 21185, our fifth nearest star, only 8.21 light years away, was found to have two probable planets orbiting it. From astrometric work undertaken by George Gatewood and his team at the Allegheny Observatory, it appeared that there were two "classical Jovians" orbiting Lalande 21185, though this was unconfirmed and had been called into question by other researchers. One had a period of about 30 years (1.6 Jovian masses, a = 10 AU) and one a period of 6 years (0.9 Jovian masses, a = 2.5 AU), it was claimed.
The G2.5V star 16 Cygni B, a sister of our Sun, 85 light years away, also had a planet orbiting it, the announcement on 23 October 1996 revealed. Planets in our Solar System all have eccentricities of less than 0.2, whereas this had an extreme eccentricity of 0.6, the highest of any known planet, so confounding all expectations. It varied from Venus-like to Mars-like distances from its parent star, had a period of 804 days and was half as big again as Jupiter.
The second Jovian-mass planet to be traced orbiting very close (0.11 AU) to its primary star was found orbiting the G8V star 55 Cancri A. Geoff Marcy subsequently announced at the Workshop on Planetary Formation in the Binary Environment at Stoney Brook on 16-18 June 1996 that a second Jovian planet was believed to exist within this system. It had a mass of ~5 Jupiters in a 15-20 year orbit. Orbiting further out is an M5 dwarf (55 Cancri B) which lies ~1150 AU away.
In 1996 another Jupiter-sized planet, similar to our sun, was found to be orbiting the 10-billion year old star Rho Coronae Borealis, in the North Crown constellation, 55 light years away. This planet could be the first to be discovered of a whole solar system orbiting Rho Coronae Borealis. It has a surface temperature of 300°C and orbits its host star every 40 days. As the first announced extrasolar planet to be discovered by the AFOE/Whipple Observatory group, Rho CrB B, at a distance of 0.25 AU (closer than Mercury is to the Sun), proved once more that Jovian-mass planets might exist in close orbits to ordinary stars.
Later that year, scientists at the Royal Observatory in Edinburgh, using a telescope 14,000 ft above sea level in Hawaii, found four young stars, all less than 100 light years away that had orbiting planets. These were Beta Pictoris, Fomalhaut, which is only 22 light years away, Vega and Epsilon Eridani, only 10.5 light years from Earth. This makes the planet orbiting it the closest yet by far, "like finding a planet in our own backyard, relatively speaking," as William Cochrane, of the McDonald Observatory at the University of Texas at Austin, said at the International Astronomical Union in Manchester in 2000. Epsilon Eridani is similar to our Sun, and the discovered gas planet lies 297 million miles from it, similar to the Sun’s distance from the asteroid belt, and far enough from its star for there to be Earth-like planets, in a habitable zone, possibly containing life, closer in. This was the first Jupiter-like planet far enough away from its star for this to be the case, and was quite an exciting development. It has a mass 0.8 - 1.6 that of Jupiter, and orbits its parent star every 7 years, about two thirds Jupiter’s own orbital period.
A Jupiter-sized planet is apparently in a 26 million-mile orbit of the star HD168443, part of a binary system in the constellation Serpens. Other examples include the 13th magnitude CM Draconis system, which has an Earth-like planet orbiting it that, because of its size (1.5-2.5 the size of Earth) and position could allow for the existence of life; and Rho Cancri.
A body orbiting Upsilon Andromedae, 44 light years (259.6 trillion miles) away in the constellation Andromeda, at 8° 33’ Taurus in 1999, was the fourth known "epistellar Jovian", discovered in June 1996. It had mass (0.60 Jovian masses), period (4.61 days), and orbital radius (0.054 AU) values that were nearly the same as those of 51 Pegasi B’s. It orbited only 4.7 million miles from its sun, a F8V star. Upsilon Andromedae was later found from variations in its wobble to have another two planets in orbit around it, both larger than the first. One has twice the mass of Jupiter and has a 242-day orbit, while the other is twice as massive, taking around 4 years to make its exaggerated elliptical orbit.
This made it the first system of planets to be discovered orbiting a parent star, and particularly exciting as the planets were of differing types. According to the astronomers who discovered them, led by R Paul Butler and Geoffrey W Marcy, "maybe we have here the Rosetta stone that will eventually lead us to understand how these planets are formed." The find implies that "planets form more easily than we ever imagined, and that our Milky Way is teaming with planetary systems." Saturn crossed Upsilon Andromeda on 11 May 1999, while Jupiter’s next crossing was on 22 March 2000.
A planet named Iota Itor B, 720 times Earth-size, in orbit around Iota Horologii, in the Southern constellation of Horologium, was announced by the European Southern Observatory in 1999. Its orbit is similar to that of Earth, taking 320 days, but is far more elliptical. At perihelion it would be close to Venus but at aphelion would be just beyond Earth’s orbit. It is at least 720 times more massive than the Earth.
Our nearest star, Proxima Centauri (discovered in 1915 by an Australian astronomer working in S Africa) is 4.2 light years away, which means that if you flew there in Concord it would take you two million years. It orbits Alpha Centauri A and B, is too faint to see with the naked eye, and is now believed to have a planet in eccentric orbit around it. Its planet, ten times the size of Jupiter, is probably a giant planet or a brown dwarf. It was the first extrasolar planet to be seen directly, on a computer-enhanced image, using the Hubble Space Telescope, rather than deduced to exist from a "wobble" around a parent star. A team led by Al Schultz of the Space Telescope Science Institute discovered it from images taken on 1 July and 13 October 1997.
Two years later, in 1999, an extra-solar planet was actually witnessed for the first time. The team of Dr Paul Butler and Geoffrey Marcy first used the Keck Telescope on Mauna Kea to detect the wobble of the star HD209458, which lies 153 light years in the constellation Pegasus. They notified Greg Henry at Tennessee State University, who focused an automatic telescope in the Patagonia Mountains on the star by remote control. The orbit of the planet brought it between the star and the telescope, causing a noticeable dimming of 1.7% each time it passed in front of the star. The planet is a gas giant, with 63% the mass of Jupiter but a radius 60% greater. Its orbit suggests it would be too hot to be able to sustain life. This direct observation finally removed any lingering ambiguity about these extra-solar planets.
Tau Boo B, an M2 dwarf, appeared in 1991 to be in an eccentric (0.91) orbit around the primary Tau Boo A in Tau Bootis as a spectroscopic binary companion, but its 2,000 year orbit was not confirmed until June 1996. The new Doppler velocity curves indicated that "C" was most likely planetary in nature by virtue of its minimum mass (about 4 times that of Jupiter). It appeared to orbit only 8 stellar radii from Tau Boo A and its planetary companion, "C". The planet was later directly observed in 1999 by astronomers led by Dr Collier Cameron at the University of St Andrews, using the telescope at La Palma in the Canary Islands. They named it The Millennium Planet. It lies 55 light years from Earth, and has a relatively high surface temperature of 1700° C. Light from the planet has been captured and used to make sense of its physical constituents. By the end of the Millennium, 28 extra-solar planets had been detected, all since 1995.
In 2000 a cluster of planet-sized objects of about 5 million years old was found free-floating through space, beyond our solar system in Sigma Orionis, an area of the Orion constellation. In the Hopi tradition this is a powerful constellation that represents a recreation of the order of creation. Astronomers were stunned to find these planets were not held in orbit by any central star. The young cool planets were too small to be brown dwarfs and emitted very little light. Had these stars "escaped" from a parent star or had they always been somehow independent from any system? These enigmatic objects raised new puzzles about the nature of space.
As extrasolar planets are relatively cool (400-1600 C), they should be much brighter in infrared light, rather than in optical light, which should make them more easily discovered with existing telescopes than had been supposed. As it is now known that water is all present in our galaxy, and that there are enormous reserves of water vapour in gas clouds in the Milky Way, many more solar systems could exist which have the potential for life forms.
The idea that the building blocks of life proliferate throughout space was given further weight in 2000 AD when glucoaldehyde, an eight-atom sugar molecule made up of carbon, oxygen and hydrogen, was found in a dust and gas cloud 26,000 light years away, near the centre of the Milky Way. "The discovery of this sugar molecule in a cloud from which new stars are forming means it is increasingly likely that the chemical precursors of life are formed in such clouds long before planets develop around the stars," said Jan Hollis, of the NASA Goddard Space Flight Centre, Greenbelt MA.
In 2001 Geoffrey Marcy, who was leading the study of 1,100 stars within 300 light years of Earth, disclosed two new discoveries, detected from their "wobbles", at an American Astronomical Society conference in San Diego, that astounded astronomers. Sir Martin Rees, the Astronomer Royal, said that both discoveries revealed the limits of our understanding of planets. Two newly observed solar systems were disclosed, both quite different from anything previously supposed by the theorists.
One system included a huge, planet-like sphere between 17 and 40 times as massive as Jupiter, making it the largest planet discovered to date. The gas monster, HD 168443c, alongside a smaller planet, orbits a star in the constellation Serpens, 123 light years away. Until this find, it was believed that planets could reach a mass only 13 times larger than Jupiter. After that, their cores would start to burn, so that they became stars or "brown dwarves". Since brown dwarves have not been observed orbiting a star as closely or as regularly as HD 168443c the discovery "calls into question the very meaning of the term ‘planet’," according to Professor Marcy. "It seems too large for a conventional planet. We frankly don’t know what name to give it. Is it a planet or a brown dwarf? Or something that formed in the protoplanetary disc but gobbled an unusual quantity of gas in that disc? We simply don’t know."
The second solar system was found around the star Gliese 876, in the constellation Aquarius, 15 light years from Earth. The star is a red dwarf, one-third Sun-size, making it the smallest star known to support a solar system. Gliese 876’s two planets have "resonant" orbits of perfect harmony: one takes 60 days to complete a circuit, and the other exactly half that time, the first known 2-to-1 resonant multi-planet system. Debra Fischer, from UCB, was reported as saying (The Times, 11 January 2001), "These two resonant planets seem to be humming in harmony. Maybe we have found the music of the planetary spheres. Now we are straining to hear the lyrics, as these planets are surely telling us something of their birth."
Another star, zeta Leporis, 70 light years away, was found to have an asteroid belt encircling it. Evidence that over half of the stars in our galaxy have Earth-sized planets orbiting them, and that life may therefore be common, was further strengthened by findings of scientists at the University of Toronto announced in February 2001, by which time 55 extra-solar planets had been identified, a total that was swiftly rising. Their method of detection had been to look for signs of burning iron on the parent star, a fuel that can be present if rocks, asteroids or planets are in orbit. Of the 640 closest stars similar to the Sun, 466 were found to be burning iron equivalent to half of the Earth’s mass. By extrapolation, 50-90% of all stars in the galaxy likewise could be.
Is it still rational to believe that our unremarkable Sun is the only star out of 100 billion or more in our galaxy, and any of the other 50 billion galaxies so far discovered, to have just one planet orbiting it, with the right mix of ingredients for life to develop?
The first known celestial event from outside our Solar System to have a direct effect on Earth was detected on 27 August 1998, when an X-ray burst from SGR 1900+14, a star in the constellation Aquila, 15,000 light years away, was detected. "The X-ray burst", wrote Marcus Chown in the New Scientist (7 November 1998, p. 62), quoted in Transit (New Series Vol. 4 No. 1), "merely confirms something that we have increasingly come to realise in the past decade or so. The Earth and its precious cargo of life are not floating in serene isolation from external events. They are very much plugged into the wider cosmos...(when) you hold up your hand (you) are looking at stardust made flesh. The iron in your blood, the calcium in your bones, the oxygen that fills your lungs each time you take a breath - all were baked in the fiery ovens deep within stars and blown into space when those stars grew old and perished. Each one of us was quite literally made in heaven. So the astrologers got it right - in spirit if not in detail. Where they slipped up is not in being too way out, but in not being way-out enough. For modern science has shown us that we are more intimately connected to the stars than anyone dared to guess."