Other 'Solar' Systems?

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 that most scientists think so. Here is some of what we do know:

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.

  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. Though it's far from certain it is believed by some that planets may already have formed around Beta Pictoris.

  

   

• 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.

  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.

   

• On 1/17/96 Geoffrey Marcy and Paul 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.

   

• 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.

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 4 known "solar 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?