I

period 4-G days period 242 days period 4 years of the mass of Jupiter twice the mass of Jupiter

4 times the mass of Jupiter of the mass of Jupiter twice the mass of Jupiter

theinnersolarsystem

Mercury

Venus

Earth

Mars

$

¥

f>

0.39 AU

073 AU

1.00 AU

1.52 AU

period

period

1 year

1.9 years

89 days

¿28 days

mass: 1 Ms

mass: 0.11 M£

mass; 0.055 ME

mass: 0.815 M{

On the same scale, a comparison of distances within the solar system and the system of the star u Andromedae (the scale of masses differs). The planetary system of u And, discovered in 1999, contains at least three planets, but there the resemblance ends. The planet (B) closest to the star is six times nearer to it than Mercury is to the Sun, but is a giant bigger than Saturn. The other two are even more massive, and have quite elliptical orbits.

Leaving aside the question of planets around pulsars, where do matters stand at present? Let us begin our answer by examining the least surprising findings. Among the two hundred exoplanets discovered by mid-2006, there are twenty-one multiple systems, and probably more if we take into account the difficulty of detecting planets smaller than Saturn. So, multi-planet systems like the Sun's are fairly common. The masses of these planets range from six Earth masses to about six Jupiters - which is not surprising, as anything smaller is undetectable. What is a surprise is that a good number of these exojupiters are very close to their stars, and the orbital distance of many of them is less than that of Mercury around the Sun. Indeed, more than forty of them are less than 0.1 AU (Astronomical Units) from their stars, giving them, according to Kepler's third law, periods of the order of just a few days. Temperatures on these planets must be typically about 1,000° C - thus their common appellation: 'hot Jupiters'. They can have little in common, though, with the original Jupiter, 5.2 AU distant from the Sun!

Yet more bizarre are the orbits of hot Jupiters. They are almost circular, while some exoplanets at greater distances, with periods measured in years, follow markedly elliptical paths. These are far more elliptical than the orbit of any planet of the solar system.

This poses a real challenge for researchers accustomed to working with a theory of the solar system that explains why orbits there are near-circular, and why gas giants are far from the Sun while terrestrial planets orbit are close in. The real difficulty lies in understanding how giants can form close to stars. An obvious solution is to imagine that they could form far out within their systems, and then migrate inwards. Theoretically this is indeed possible, but two questions then arise. What causes the migration to cease, preventing the hot Jupiters from falling into their stars? And why, in our solar system, have Jupiter and Saturn remained where they are?

The solar system does seem to be quite an exceptional place. Will we one day find an exoEarth? Just finding a planet of terrestrial mass is not enough: it must not be too near (like Mercury or Venus) or too far (like Mars) from its star. If other planetary systems are so different from our own, we may not find it easy to discover a twin of the Earth!

The system of the star HD 74156, discovered in 2004, comprises two planets comparable to Jupiter in mass. Their orbits (shown in yellow) are very elliptical, compared with orbits in the solar system (in red).

The system of the star HD 74156, discovered in 2004, comprises two planets comparable to Jupiter in mass. Their orbits (shown in yellow) are very elliptical, compared with orbits in the solar system (in red).

Solar system

Mass (Me)

Semimajor axis (AU)

e*

Mercury

0.055

0.387

0.206

Venus

0.949

0.723

0.007

Earth

1

1

0.017

Mars

0.107

1.52

0.093

HD 74156

Mass (Me)

Semimajor axis (AU)

e*

b

0.88

0.73

0.54

c

1.63

1.16

0.41

*e - orbital eccentricity; for a circular orbit, e = 0

*e - orbital eccentricity; for a circular orbit, e = 0

Measuring exoplanets

The solar system is unlike any other known planetary system, but nevertheless is used as a standard, with jupiter and the Earth as reference planets. Masses of exoplanets are measured in Jupiter masses (M,), although those of some smaller bodies are expressed in Earth masses (ME). Orbital radii are logically given in jupiter radii (Rj), and distances from the central star in Astronomical

Units. The orbital periods of exoplanets are given in days or years.

Mass of Saturn: 95.1 Me

1 Astronomical Unit (AU) = 149,600,000 km

Radius of Jupiter: 1 Rj = 142,800 km - 11.2 times the Earth's equatorial radius

Was this article helpful?

0 0

Post a comment