Gravitational microlensing

4.3 Gravitational microlensing

The detection of Earth-mass planets by the transit method still (in 2007) lies in the future, but there is a third method which has the potential to achieve this now – and has already detected a 5 Earth-mass planet. In Chapter 2 we saw how Einstein’s General Theory of Relativity was proven by the observed movement of star’s positions due to the curvature of space close to the Sun. This effect gives rise to what is called gravitational lensing, specifi cally gravitational microlensing as the effects are on a very small scale. In the same way that a convex lens can con- centrate the light from a distant object into the eye and so make it appear brighter, if a distant star passes behind one of intermediate distance, the brightness of the distant star will undergo a temporary increase which can last for many days. The peak brightness can be up to 10 times (2.5 magnitudes) that normally observed. More than a thousand such events had been observed by the end of 2007.

If the lensing star has a planet in orbit around it, then that planet can produce its own microlensing event, and thus provide a way of detecting its presence. For this to be observed, a highly improbable alignment is required so that a very

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large number of distant stars must be continuously monitored in order to detect planetary microlensing events. Observations are usually performed using net- works of robotic telescopes (such as those forming the OGLE collaboration) which continuously monitor millions of stars towards the centre of the galaxy in order to provide a large number of background stars.

If one of the telescopes fi nds that the brightness of a star is increasing, then the whole network, spaced around the world for continuous observation, will provide unbroken monitoring. The presence of a planet is shown by a very short additional brightening appearing as a spike on the fl anks of the main brightness curve.

On January 25, 2006, the discovery of OGLE-2005-BLG-390Lb was announced (Figure 4.8). This planet is estimated to have a mass of ∼5.5 Earth masses and orbits a red dwarf star which is around 21500 light years from Earth, towards the centre of the Milky Way Galaxy (Figure 4.9). The planet lies at a distance of

2.6 AU from its sun. At the time of its discovery, this planet had the lowest mass of any known extra-solar planet orbiting a main sequence star. This record may still hold unless the mass of Gliese 581c, discovered in April 2007 by the radial

velocity method, is found to have a mass very close to its minimum mass of 5 Earth masses. By the end of 2007, four extra-solar planets had been discovered using the microlensing technique.

A disadvantage of the method is that the chance alignment that allowed the lensing event that led to the planet’s detection is highly unlikely to be ever repeated. Also, the detected planets will tend to be many thousands of light years

Figure 4.8 Observations by the OGLE consortium showing the microlensing caused by a planet

Extra-solar Planets

Figure 4.9 Artist’s impression of the 5.5 Earth-mass planet which circles its red dwarf star

with a period of 10 years. The planet has surface temperature of ∼220°C below zero. It is likely

to have a thin atmosphere with a rocky core buried beneath a frozen icecap. Image: ESO press release March 2006.

away, so making any follow-up observations by other methods virtually impos- sible. However, if enough background stars can be observed over long periods of time the method should fi nally enable us to estimate how common Earth-like planets are in the galaxy.

In early 2008, the gravitational microlensing method detected two gas giant planets, similar to Jupiter and Saturn, orbiting a star 5000 light years away in a planetary system with striking similarities to our own Solar System. The discov- ery suggests that giant planets do not live alone but are more likely to be found in family groups. The mass of the nearer planet is 0.71 times that of Jupiter and it lies 2.3 times as far from its host star as the Earth is from the Sun. The second planet is less massive; 0.27 times the mass of Jupiter, and twice as far away from the host star.

Despite their host star only being half as massive as the Sun the planetary sys- tem otherwise bears a remarkable similarity to our Solar System. Both the ratio of the masses of the two giant planets (close to 3:1) and the ratio of their distances from the host star (1:2) are remarkably similar to those of Jupiter and Saturn. The ratio between the orbital periods of 5 years and 14 years, respectively, also closely resembles that between Jupiter and Saturn (2:5). The newly discovered system resembles our own Solar System more closely than any previously observed. Whilst there are more than 250 planets now known to be orbiting other stars, there are only about 25 solar systems known to have multiple planets; this num- ber will surely rise as smaller planets fall within our detection methods.

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