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The stars visible to the unaided eye, particularly the brighter ones, have names stretching back into antiquity - Canopus, Polaris, Rigel, Sirius, Vega, and so on. Such stars are also designated by a Greek lower case letter followed by the name of the constellation in which the star appears, roughly in order of decreasing visual brightness, with a (alpha) almost always denoting the brightest star in the constellation, p (beta) almost always denoting the next brightest star, and so on through the Greek alphabet, ending with ro (omega). Thus, Vega, the brightest star in the constellation of Lyra (the harp) is also called a Lyrae. Note that the name changes to the genitive case - Lyra to Lyrae ("lie-ree"). This system has been in operation since it was introduced by the German astronomer Johann Bayer (1572-1625) in the early years of the seventeenth century.

The great majority of stars have no individual names, but the brighter ones are still designated by a Greek letter followed by the constellation name, such as s (epsilon) Eridani, the fifth brightest star in the constellation Eridanus (the river).

The trouble with the Greek alphabet is that it only has 24 letters, yet there are far more stars in each constellation than this that are visible to the unaided eye, and hugely more that are visible through even a small telescope. There were several attempts to extend the lettering system, but today we use only the Greek letters. To include more stars, numbering systems were introduced, and one still in widespread use was brought in by the British astronomer John Flamsteed (1646-1719). The final version of his catalog, containing nearly 3,000 stars, was published in 1725. Within each constellation, stars are numbered in the order of increasing celestial longitude (called right ascension), as these were in about 1700, and not according to brightness. Examples are 51 Pegasi (star 51 in Pegasus, the flying horse) and 47 Ursae Majoris (star 47 in Ursa Major, The Great Bear).

Also common today is a numbering system published in 1914-1918 by the US astronomer Annie Jump Cannon (1863-1941) and co-workers at Harvard College Observatory. It is named the Henry Draper Catalog after the man whose widow financed the catalog. Stars are again numbered in the order of their celestial longitude in the sky, but are not sorted into constellations, and include fainter stars than in Flamsteed's catalog. There are nearly 360,000 stars in this catalog - HD1, HD2, etc.

Stars also appear in various more specialized star catalogs, for example, there is the Gliese ("glee-za") catalog of all stars within about 245 light years known in 1991. Stars in this catalog are denoted by G or Gl followed by a number. Yet another catalog is devoted to variable stars.

FIGURE 11.2 The variation in radial velocity of the star Tau1 Gruis (HD216435), at its discovery in 2002. It is about 109 light years away, has a mass about 1.25 times that of the Sun, and might be at the end of its main sequence lifetime. Its planet has a minimum mass 1.49 times that of Jupiter, and moves in an orbit 2.7 AU from its star, with an eccentricity of 0.34.

1998 1999 2000 2001 2002 year

FIGURE 11.2 The variation in radial velocity of the star Tau1 Gruis (HD216435), at its discovery in 2002. It is about 109 light years away, has a mass about 1.25 times that of the Sun, and might be at the end of its main sequence lifetime. Its planet has a minimum mass 1.49 times that of Jupiter, and moves in an orbit 2.7 AU from its star, with an eccentricity of 0.34.

Almost all of the 273 non-pulsar planets have been discovered using Doppler spectroscopy. The few that have not have been discovered by transit photometry, gravitational microlensing, or direct imaging. Transit photometry and direct imaging give no information about the mass of the orbiting object, but in many cases, planetary masses of the object have subsequently been determined by Doppler spectroscopy.

Figure 10.11 showed radial velocity data for three stars, along with fitted curves. Figure 11.2 shows another example, with a smaller radial velocity range, and a long period. I have also added vertical bars to indicate the uncertainties in the radial velocity measurements. The radial velocity of the center of mass of the system has been subtracted. The data on the star and its only known planet are given in the Figure caption.

The first discovery by another method was in 2003, of a planet orbiting the star OGLE-TR-56. This star was un-named before the discovery, and its name carries that of the survey - OGLE (Optical Gravitational Lensing Experiment, Section 9.3). In spite of the survey name, the discovery was made by transit photometry (Figure 11.3). This is because a survey looking for the light amplification in gravitational microlensing can also detect changes in apparent stellar brightness due to a transit. That OGLE-TR-56 b has planetary mass has been confirmed by Doppler spectroscopy. See the Figure caption for details.

OGLE-TR-56 is about 5,000 light years away, which explains why the light curve is very "noisy" - compare Figure 11.3 with the HST curve for HD209458 in Figure 9.2, which is only about 150 light years away. Some of the few other planets discovered by transit photometry, and all of the few discovered by

5% change in apparent brightness of star

5% change in apparent brightness of star

FIGURE 11.3 The light curve of the 1.17 solar mass main sequence star OGLE-TR-56, about 5,000 light years away. The dip is due to its transiting planet. The planet's actual mass is 1.29 times the mass of Jupiter and 1.3 times Jupiter's radius. It orbits at 0.0255 AU with a period of 1.211909 days.

FIGURE 11.3 The light curve of the 1.17 solar mass main sequence star OGLE-TR-56, about 5,000 light years away. The dip is due to its transiting planet. The planet's actual mass is 1.29 times the mass of Jupiter and 1.3 times Jupiter's radius. It orbits at 0.0255 AU with a period of 1.211909 days.

gravitational microlensing, are also far away. The very great majority of the other stars with planets are within 300 light years, which is only about 0.3% of the diameter of the disc of the Galaxy in which we live (Section 7.1). Therefore, all these other exoplanets are in our cosmic backyard. Why so close?

The reason for their proximity is because at close range we receive more stellar radiation, and so all detection methods have an easier time. This is even the case for Doppler spectrometry, even though the Doppler shifts are independent of stellar distance. This is because the closer the star the more distinct are its spectral lines, and the tiny Doppler shifts caused by planetary mass bodies are consequently easier to measure.

Some exoplanets discovered by Doppler spectroscopy have since been detected by transit photometry. Recall that to observe a transit the planet's orbit has to be presented to us very nearly edge-on. Exoplanet orbits are orientated randomly on the sky. Calculations show that in this case, out of the 250 or so Doppler discoveries, we can only expect to observe transits for approximately four, which is about what we have.

One system discovered by Doppler spectroscopy, that of the star Gliese 876, has since been detected astrometrically by the HST. This detection was aided by the low mass of Gliese 876, 0.32 solar masses, and its proximity, 15.4 light years, leading to a large astrometric signal. Another astrometric confirmation has been made of the low mass companion of HD89744, by the UK Infrared Telescope on Mauna Kea, but its mass is near the 13 Jupiter mass boundary between giant planets and brown dwarfs, so its planetary status is not confirmed. No exoplanets have yet been discovered by astrometry.

Only one exoplanet has so far been detected for certain by imaging, but this orbits the brown dwarf 2M1207 rather than a main sequence star. The planet has a mass roughly five times that of Jupiter, and orbits its star well beyond the HZ. The young K dwarf AB Pic has an object at a projected distance of about 275 AU from its star, but its mass, 13-14 times that of Jupiter, places its planetary status in question. The mass of GQ Lupi b (Section 8.3) is still very uncertain - it could be anywhere in the range 1-40 Jupiter masses.

Let us now look at the discovered exoplanetary systems in more detail, including the stars at their hearts, the planetary orbits, the exoplanets as bodies, and the ways in which the various systems might have formed. Remember that I am excluding the few pulsar planets.

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