First attempts at detection and first disappointments

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In the nineteenth century, astronomers began to find evidence of the presence of smaller companions to stars when they detected slight periodic motions of those

Edward Emerson Barnard (1857-1923) Piet van de Kamp (1901-1995). In 1964

was one of the greatest observers of his day. van de Kamp announced his discovery of a

He discovered the fifth moon of Jupiter, planet orbiting Barnard's Star, but in 1973 it

Amalthea, in 1892, and is also remembered was demonstrated that the star has no for the red dwarf star which bears his name, companion.

Edward Emerson Barnard (1857-1923) Piet van de Kamp (1901-1995). In 1964

was one of the greatest observers of his day. van de Kamp announced his discovery of a

He discovered the fifth moon of Jupiter, planet orbiting Barnard's Star, but in 1973 it

Amalthea, in 1892, and is also remembered was demonstrated that the star has no for the red dwarf star which bears his name, companion.

2.3 Barnard's Star: a disappointment 19

2.3 Barnard's Star: a disappointment 19

The Sproul Observatory at Swarthmore College, near Philadelphia. Here the Dutch astronomer Piet van de Kamp carried out studies of the motion and oscillations of Barnard's Star.

stars in relation to other nearby stars. The German astronomer-mathematician Friedrich Bessel was the first to detect such a body in this way - in this case the companion of Sirius. Sirius's partner, an extremely dense white dwarf star, was massive enough to affect the motion of its primary (Sirius A) without necessarily being detected visually.

A century later, improved techniques allowed astronomers to search for even smaller companions: for example, 'brown dwarf stars or giant planets. By the 1940s the search for giant exoplanets was beginning. Patience was the key. Remember that within our solar system the periods of revolution of the giant planets around the Sun vary from 12 years in the case of Jupiter to 164 years in the case of Neptune. After several reports (later unconfirmed) of companions detected in the vicinities of the stars 61 Cygni and 70 Ophiuchi, a serious candidate took the stage when the Dutch astronomer Piet van de Kamp announced the discovery of a planet orbiting Barnard's Star. This star is a probable target for exoplanetary research, as it is the fourth closest star to the Sun. In 1964, on the basis of photographs taken over a twenty-year period, van de Kamp described the characteristics of the planet: its mass was 1.6 times that of Jupiter, and the period of its elliptical orbit was 24 years. As more data were produced, van de Kamp was able to refine his description, and in 1982 he concluded that there were two companions of Barnard's Star, with masses less than Jupiter's, and with periods of 12 and 20 years.

However, this spectacular result did not withstand the test of time. In 1973 a study revealed systematic errors in the performance of van de Kamp's Sproul Observatory telescope, which might have been responsible for the observed oscillations. In the same year the American astronomer George Gatewood published an independent study on the motion of Barnard's Star, and concluded that it had no companion. 'Barnard's Planet' may have had its day, but van de Kamp certainly counts as a pioneer in the field of research which is in outburst. The current avalanche of discoveries since planets were detected around a pulsar in 1992, has seen the first successes of the velocimetric method in 1995 and, in 1999, the first detection of an exoplanet by transit observations.


Planets are found near dying stars

In recent times, astrometrists have sought exoplanets orbiting Sun-like stars, in the hope that they might one day find a world capable of supporting life. In the 1970s, however, another kind of star exercised their minds: pulsars (pulsating stars) had recently been discovered by radio astronomers. These objects represent the final stage in the evolution of some massive stars that have exhausted their supplies of hydrogen and helium. Heavier elements are later synthesised, and they become supergiant stars, and explode as supernovae, ejecting most of their material into space. The central remnant collapses upon itself to form an extremely dense neutron star - its mass comparable to that of the Sun, but packed into a volume about 20 km in diameter! Neutron stars rotate very rapidly, with periods of the order of 1 second. Pulsars exhibit very strong (synchrotron) radio emissions, keeping time with their rotation. Radio astronomers detect these highly regular pulses: the signature of pulsars.

Detecting exoplanets around pulsars

Pulsar timing is the most sensitive method for detecting pulsar planets. It involves the study of variations in the pulsar's period, expressed in milliseconds. These variations are of the order of 1.2 [Mp] [P]2/3, where P is the period of revolution of the planet in years and Mp is the mass of the planet in Earth masses, assuming a circular orbit. Since this method is extremely precise, tiny variations in the period can be detected: for example, near PSR 1257+12 a companion of 0.020 Earth masses (1.6 times the mass of the Moon) has been found. By this method, even 'exoMoons' are detectable. Note that Lhe sensitivity of this method does not explicitly depend upon the distance of the pulsar. However, difficulties remain, as the signals from more distant pulsars are weak, and timings of them are less accurate.

2.4 Planets around pulsars 21

0.020 0 4.3 0 3.9 0

.19 25.3 .36 66.5 .46 98.2

PSH 12S7 12 b

PSH 125? 12 d

* f •


PSR 1ZS? 1? 3 PSR 125? lie

M (Me)

Semi-major axis (A.U.)

Period (days)

0.55 (Mercury)



0.949 (Venus



1 (Earth)



The planetary system of PSR 1257+12. Above left: at least three planets orbit PSR 1257+12. The masses of the two largest (b and c) are 4.3 and 3.9 times that of Earth, while the smallest (a) is a little more massive than the Moon. Above right, and below: planets a, b and c have near-circular orbits very close to the star, at distances smaller than that of Mercury from the Sun. If the central star were not a pulsar, this would be the exoplanetary system which most resembles the solar system! Another pulsar with planets is PSR 1 620-26. Its system - with one planet of 2.5 Jupiter masses orbiting at 40 AU - is quite unlike that of PSR 1257+12.

Very accurate measurements of radio signals from pulsars provided a new method in the search for possible exoplanets, the presence of which might be revealed by variations in the periods of these stars. Several 'detections' were announced, but it later transpired that they were caused by the pulsars' internal instabilities. Then, in 1991, British astronomer Andrew Lyne published his discovery of a Uranus-sized companion to PSR 1829-10 - a pulsar 30,000 light-years away. The orbital period of the planet was calculated to be 6 months. But disappointment continued to stalk the astronomers. The observed effect was not due to the presence of a planet, but to an inaccurate 'too circular' correction for the elliptical motion of the Earth around the Sun. Once this was corrected, the irregular periodicity, and the planet, disappeared.

Finally, as Lyne came to terms with the disappointment of a negative result, the long search for a 'pulsar planet' came to an end. In 1992, Polish astronomer Aleksander Wolszczan announced the discovery of two planets orbiting pulsar PSR 1257+12. This is a very different pulsar from that previously mentioned. Its period is an incredibly short 0.0062 seconds, placing it in the category of millisecond pulsars. These have life stories which are not at all the same as those of classical pulsars. Millisecond pulsars are much older, having existed for hundreds of millions of years. Their peculiar nature can be explained thus. They are the end product of a pair of stars - one normal and the other a neutron star. As the normal star ages it dilates, transferring momentum to the neutron star and spinning it up. How can such an object possess planets? This must be a result of the accretion of stellar matter into a disk around the neutron star, and it is within this disk that planets are later formed. Such worlds are doubtless very different from the kind of planets with which we are familiar.

It was later discovered that PSR 1257+12 has three planets, with masses between 0.02 and 100 times the mass of the Earth. Apart from a planet 2.5 times the mass of Jupiter, companion to pulsar PSR 1620-26 and discovered in 1999, no other 'pulsar planets' have yet been detected near millisecond or standard pulsars. This suggests that they are rare objects - especially since detection methods are very sensitive.

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