Other Planetary Systems?

Proplyd

Are there planets orbiting other stars beyond our solar system? We do not know for sure, but with the recent discoveries about 51 Pegasi, 70 Virginis and 47 Ursae Majoris the weight of evidence is now so strong that only a "devil's advocate" denies the conclusions. Here is some of what we do know (this is somewhat incomplete; please see the references below for more info):

Facts

  • Three small bodies have been found in orbit around the pulsar PSR 1257+12. They have been designated "PSR1257+12 A, ..B, and ..C". One is about the size of the Moon, the other two are about 2 to 3 times the mass of Earth.

    They were discovered by measuring variations in the pulsation speed of the pulsar which can be interpreted as gravitational effects of three small planets. The original observation has been confirmed but, of course, no direct images have been made -- that is way beyond the capabilities of our best telescopes.

    These planets are believed to have formed after the supernova that produced the pulsar. The present planets would have originally been within the envelope of the progenitor star and therefore wouldn't have stood much chance of surviving the supernova explosion, and wouldn't have remained in circular orbits after the explosion.

    Several decades of timing data on the pulsar PSR 0329+54 (PKS B0329+54) by Tatiana Shabanova (Lebedev Physics Institute) shows evidence of a planet with a 16.9 year period and mass greater than 2 Earth masses.

    But, while the evidence for these is pretty good, they aren't really what we're looking for when we talk about 'solar systems'.

  • It has been known since 1983 that the star Beta Pictoris is surrounded by a disk of gas and dust. Spectra of Beta Pictoris show absorption features which are currently believed to be due to cometary like clouds of gas occultating the star from the debris left over from planetary formation. Though it's far from certain it is believed by some that planets may already have formed around Beta Pictoris.

    HST has observed Beta Pictoris (right) and found the disk to be significantly thinner than previously thought. Estimates based on the Hubble image place the disk's thickness as no more than one billion miles (1600 million kilometers), or about 1/4 previous estimates from ground-based observations. The disk is tilted nearly edge-on to Earth. Because the dust has had enough time to settle into a flat plane, the disk may be older than some previous estimates. A thin disk also increases the probability that comet-sized or larger bodies have formed through accretion in the disk. Both conditions are believed to be characteristic of a hypothesized circumstellar disk around our own Sun, which was a necessary precursor to the planet-building phase of our Solar Systems, according to current theory.

    More recent HST observations have shown the disk to be slightly warped as might be expected from the gravitational influence of a planet. This has been confirmed by observations at ESO.

  • Recent observations at radio wavelengths of a gas cloud known as Bok Globule B335 have produced images of material collapsing onto a newly born star (only about 150,000 years old). These observations are helping to understand how stars and planets form. The phenomena observed matches the theory of the formation of the solar system -- that is, a large gas cloud collapsed to form a star with an attendant circumstellar disk in which, over time, planets accreted from the matter in the disk and orbited the Sun.

  • The IRAS satellite found that Vega had too much infrared emission, and that has been attributed to a dust shell (with a mass of maybe Earth's moon).

  • Observations of the very nearby Barnard's Star were once thought to be evidence of gravitational effects of planets but they now seem to have been in error.

  • The star Gl229 seems to contain a 20 Jupiter mass object orbiting at a distance of 44 AU. An object this large is probably a brown-dwarf rather than an ordinary planet.

  • What may be the first discovery of a planet orbiting a normal, Sun-like star other than our own has been announced by astronomers studying 51 Pegasi, a spectral type G2-3 V main-sequence star 42 light-years from Earth. At a recent conference in Florence, Italy, Michel Mayor and Didier Queloz of Geneva Observatory explained that they observed 51 Pegasi with a high-resolution spectrograph and found that the star's line-of-sight velocity changes by some 70 meters per second every 4.2 days. If this is due to orbital motion, these numbers suggest that a planet lies only 7 million kilometers from 51 Pegasi -- much closer than Mercury is to the Sun -- and that the planet has a mass at least half that of Jupiter. These physical characteristics hinge on the assumption that our line of sight is near the planet's orbital plane. However, other evidence suggests that this is a good bet. A world merely 7 million km from a star like 51 Pegasi should have a temperature of about 1,000 degrees Celsius, just short of red hot. It was initially thought that it might be a solid body like a very big Mercury but the concensus now seems to be that it is a "hot Jupiter", a gas planet formed much farther from its star that migrated inward.

    These observations have now been confirmed by several independent observers. And there is some evidence for a second planet much farther out that is not yet confirmed.

    [ The 5.5-magnitude 51 Pegasi is easily visible in binoculars between Alpha and Beta Pegasi, the western pair of stars in the Great Square of Pegasus. The star's equinox-2000 coordinates are R.A. 22 hours 57 minutes, Dec. +20 degrees 46 minutes. ]

  • On 1/17/96 Geoffrey Marcy andPaul Butler announced the discovery of planets orbiting the stars 70 Virginis and 47 Ursae Majoris. 70 Vir is a G5V (main sequence) star about 78 light-years from Earth; 47 UMa is a G0V star about 44 light-years away. These were discovered using the same doppler shift technique that found the planet orbiting 51 Pegasi.

    The planet around 70 Vir orbits the star in an eccentric, elongated orbit every 116 days and has a mass about nine times that of Jupiter. Using standard formulas that balance the sunlight absorbed and the heat radiated, Marcy and Butler calculated the temperature of the planet at about 85 degrees Celsius (185 degrees Fahrenheit), cool enough to permit water and complex organic molecules to exist. The star 70 Vir is nearly identical to the Sun, though several hundred degrees cooler and perhaps three billion years older.

    The planet around 47 UMa was discovered after analysis of eight years of observations at Lick Observatory. Its period is a little over three years (1100 days), its mass about three times that of Jupiter, and its orbital radius about twice the Earth's distance from the Sun. This planet too probably has a region in its atmosphere where the temperature would allow liquid water.

  • As of April 1996, Drs. Marcy and Butler have discovered yet another planet this time around the star HR3522 (aka Rho 1 Cancri, 55 Cancri) about 45 light years from the Earth. The planet is estimated to be about 0.8 Jupiter masses. It is likely that several more planets will show up in the initial set of 120 stars that they have monitored.

  • Several more extra-solar planets have now been discovered by the Butler/Marcy method. It seems likely that there are a very large number of such planets out there.

  • Another extra-solar planet has been discovered orbiting 16 Cygni B. But unlike all other previously known planets this one has a very large orbital eccentricity (0.6); its orbit carries it from a closest distance of 0.6 AU from its star to 2.7 AU. This calls into question many theories of planetary formation.

  • Detecting extra-solar planets directly is very difficult. Even the Hubble Space Telescope wouldn't be able to image planets at the expected sizes and distances from their suns.

    What HST did find were disks of matter around stars seen in silhouette against the Orion Nebula (called 'proplyds', for 'proto-planetary disks' (right). This is great evidence for how common these objects are, but the scale is way too small to say anything directly about planets there. More detailed HST images are now available, too.

  • Nevertheless, it might be possible to detect the infra-red radiation of very large planets (Jupiter-sized or more) in some circumstances.
  • By a stroke of good luck, HST has taken an image of what appears to be a planet escaping from a double star system. See the 1998 May 28 announcement. If this is confirmed, the existence of extrasolar planets will be undeniable.

More about other planetary systems

Open Issues

  • It is likely that more planetary systems will be discovered using the methods that found 51 Pegasi, 70 Virginis and 47 Ursae Majoris. Exciting times are ahead!
  • The 51 Pegasi planetary system is quite different from our solar system. But the 70 Virginis and 47 Ursae Majoris systems appear to be more "normal". With several known planetary systems it is now possible to make more general statements and to better test theories of planetary formation.
  • None of the extra-solar planets discovered so far are at all similar to the Earth, as expected given our current methods. How can we get more evidence about extra-solar Earth-like planets? Will the ExNPS program get funded?
  • How do you get a planet with a large eccentricity?
  • [abstract of the above paper by Dr. Fraknoi]: Interest among astronomers in the detection of extra-solar planets is accelerating with the growing realization that it may soon be technically feasible. The ongoing renaissance in telescope construction and the anticipated launches of new space platforms are encouraging many scientists to review and improve the means by which planets can be discovered. The direct detection of the light from a distant planet would be the most compelling means of discovery and to gauge the feasibility of various search strategies, astronomers have traditionally used the current Jupiter as a benchmark planet. However, in principle, extra-solar giant planets (EGPs) can have a wide range of masses and, hence, can be significantly brighter than Jupiter. Furthermore, the maximum mass a planet can have is not known a priori, and observations will be needed to determine it. We predict the optical and infrared fluxes of EGPs that searches in the next few years may reveal.

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