Evening Lecture

Thursday 24th April 2008 – 8 pm


“Our Solar System & Planets Around Other Stars”


Professor David Hughes



Idea developed by Sagan that we live in a “normal” solar system with reasonably well understood physics governing planet formation and that as such our galaxy must contain very many similar solar systems. Sagan estimated 1 million instances of intelligent life in our galaxy – where are they?

Several ways to detect extrasolar planets. Early ones were found from small errors in the regularity of pulsar pulses - planets around a neutron star are certainly not normal!

Most successful method is to measure a Doppler shift in the star’s light as a result of its travel around a system centre of mass that is displaced from its own centre because of planetary masses. Jupiter contains 75% of the planetary mass in our system, Jupiter induces a motion in the Sun of 12m/s. Not easy to spot in light moving at 3exp8 m/s. Best induced speed measurements so far about 50m/s. This leads to a measurement bias in favour of planets with greater than Jovian masses and closer than Jupiter to their parent stars.

Similarly transit measurements (drop in a star’s light as a planet passes in front of it) also favour large and close planets - Jupiter would cause a 1% drop in the Sun’s light for 29hours every 11˝ years – not easy to spot



The majority of the ~300 planets found are heavier than Jupiter and inside Jupiter’s orbit (5 AU or five times the earth sun distance). In the diagram the planets with Earth like or sub Earth masses tend to be pulsar planets, the very low mass planet is what has been described as a comet orbiting the first pulsar to have a planet discovered and which has three other claimed planets.

The problem with the above for planetary scientists is that if our solar system is normal with well understood planetary physics how can what are presumably gas giants be explained with orbits inside that of mercury?

Even allowing for measurement bias our solar system looks a little less normal!