OTHER WORLDS THAN OURS

Doug Daniels

When the Hampstead Scientific Society was formed, a little over a hundred years ago, our Solar System was regarded as unique. Although, in some quarters, there was speculation regarding the possible existence of other 'solar systems', the stars were regarded as being far too remote for any meaningful investigation. It is only in the last two decades that instrumentation and techniques have been developed to such an extent that we are now beginning to discover that planetary systems surrounding other stars may in fact be quite common. Astronomers have coined the term 'exoplanets' to describe bodies orbiting distant stars and more and more are being discovered year on year. At the time of writing, 270 exoplanets are known to exist and we can confidently expect this number to rise with the passing of time.

Searching for exoplanets is a difficult business and one that requires very sensitive instrumentation and much patience. Compared to the stars that they orbit, planets are relatively small bodies. As they shine by reflected light they are dim, by a factor of about 10¹⁰ in visible wavelengths, compared to their primaries. This makes direct observation almost impossible so astronomers have to apply more subtle techniques in order to detect their presence.

Many exoplanets have been detected by observing a star's wobble as it is pulled about by the orbiting planet's gravitational field. This is observed by measuring the shift in the star's position relative to more distant stars. This astrometrical technique requires very delicate measurements and can only detect massive planets. A refinement employs measurement of the Doppler shift in the star's spectrum. This shift in the wavelength of the visible light is proportional to the ratio of the masses of star and planet but it can be a lengthy process, as at least one complete orbit of the planet must be observed to obtain meaningful results.

Another technique is to observe the dimming of the light from a star when a large planet transits the star. This can only work in a small number of cases, when planets transit their primaries in our direct line of sight. Nevertheless, it is able to identify a Jupiter sized planet transiting a Sun sized star. During such a transit, the light from the star could decrease by as much as 1%. This amount of dimming is easily detectable with relatively small earthbound telescopes employing state-of-the art photometry. Even so it is necessary to check out a large number of possible candidates, seeking the few that might exhibit periodic dimming.

Another more complex technique makes use of the effect of gravitational lensing. In this case the light from a more distant star in the field of view, is bent by the gravitational field of the star/planet system and periodic changes in this distortion can infer the existence of a large planet orbiting the star.

All of these techniques can be exploited to reveal much more information about exoplanets. The Doppler technique can describe the planet's size, its orbital period, inclination and eccentricity and its mass. The transit method provides the planet's radius and orbital inclination. Combining these techniques provides the planet's mass, size and density, which in turn provides a clue as to the possible composition of the planet.

Most of the exoplanets so far discovered turn out to be 'gas giants' with masses between 150 and 1,000 times that of Earth with radii somewhere between 0.8 and 1.7 times that of Jupiter and many appear to be less dense than current theory concerning planetary formation expects. A possible explanation proposes that these less dense planets must be very hot bodies and have been designated as 'hot Jupiters'. They orbit very close to their parent stars unlike our own Jupiter, and these low-density planets may consist mainly of hydrogen and lack any substantial heavy elements in their cores. Such a planet is that known as HD 189733b, which has been closely observed with the NASA infrared Spitzer space telescope. This planet orbits its parent star at a distance of .03 AU, that is just 3% of the Earth/Sun distance. Because of the close proximity of the parent star, the planet's axial and orbital rotation periods have become synchronized so that it always keeps the same face turned towards the star. This planet has been observed being occulted by its parent star and emerging on the other side of its orbit. This has enabled Spitzer to measure the temperature difference between the 'day' and 'night side' of the planet, and found it to be about 230 degrees C. It also found asymmetry of the hottest and coldest regions, which do not correspond to points exactly facing towards and away from the star. From this we can conclude that the planet possesses a very turbulent atmosphere with strong currents and high wind speeds which circulate the heat. Spectroscopic observations have revealed the presence of atomic sodium, hydrogen and water in the upper atmosphere of HD 189733b and water has also been detected on two other 'hot Jupiters' -- TrES-1 and HD209458b.

It seems possible that a whole menagerie of exoplanet types can exist, ranging from hot and cool gas giants to 'super earths'- rocky planets with 5-10 times Earth's mass. At least two such planets orbit the star Gliese 581, a small cool star 75 times less luminous than the Sun. Their orbital positions suggest that they may exist in a region with a temperature range between 460° C to minus 55° C similar to temperatures on Venus and Mars. It is possible that these 'super earths' may be ocean planets having been formed in the cooler regions of the protoplanetary disk, and subsequently being forced to migrate inwards by a third Neptune-mass planet that also orbits the star.

Another interesting exoplanet recently discovered is that found to be orbiting the young star TW Hydrae, which is less than 10 million years old and situated 180 Lt.Yrs. from Earth. The planet -- TW Hya b. orbits its parent star in just 3.56 days at a distance of only 3.7 million miles from its primary and has a mass 9.8 times that of Jupiter. The planet appears to have swept clean a region of the protoplanetary dust disk up to 5.6 million miles from the protostar and may still be accreting material. At a mere 10 million years old, TW Hya b is the youngest planet to be discovered so far and could help to refine our understanding of the mechanism of planet formation from the protoplanetary disks surrounding young stars in the very early stages of planet formation.

So far our instruments and observational techniques have only succeeded in detecting massive planets, so what are the chances of discovering smaller Earth-like rocky planets? The search has already begun with space missions such as COROT, launched in Dec. 2006 and NASA'S Kepler probe due for launch in 2009. As imaging and detection techniques become more refined we can look forward to more significant discoveries in the not too distant future.

It is just 13 years since the discovery of the first exoplanet and new discoveries are being made almost on a monthly basis. We now know for certain that our own Solar System is certainly not unique and that throughout the Universe there are certainly many 'other world than ours'. Many of these 'new worlds' will be vastly different from our own but considering the potential numbers of 'extra Solar' planetary systems that must exist in such a vast universe, it is no great stretch of the imagination to envisage many Earth-like planets and some perhaps capable of supporting life. But what that life might be - well, that can certainly stir the imagination.

Source & Further reading: Nature Vol. 451/3 Jan. 2008.

1. Extrasolar planets. Dimitar D. Sasselov.
2. A young massive planet in a star-disk system. J. Setiawan, Th. Henning, R. Launhardt, A. Muller, P. Weise & M. Kurster

MORE DEBATE ON GLOBAL WARMING

Doug Daniels

The scientific community appears to be polarising into two distinct factions: those who think that climate change is all 'our fault' and those who don't.

I think it fair to say that there is no longer any doubt that the phenomenon is 'real' and that there is certainly more than enough evidence that average pan-global temperatures are at present increasing with time. Our own meteorological records at Hampstead Observatory, which span almost a century from the same site, indicate that from Hampstead at least, average temperatures have risen by about 1° Celsius during the last 100 years. While there is almost total agreement that this is happening, there is little or no agreement as to the cause.

The opinions of the two main factions are that (1) the problem is caused by mankind contaminating the atmosphere by burning fossil fuels and consequently increasing the concentrations of 'green-house gases' or (2) that climate change is a natural cyclical phenomenon and has little or nothing to do with any of the activities of mankind.

Over geological time, the Earth's climate has changed many times. There have been periods when the planet has cooled down dramatically causing catastrophic species extinctions and at other times it has heated up producing the same results. These episodes have occurred long before the evolution of Homo Sapiens. It is argued that such fluctuations in climate must occur given the mobility of the continents caused by plate tectonics and the variations in Earth's axial tilt, axial direction and perihelion distance (due to variations in orbital elipticity) - the famous Milancovic Cycles.

But there is also a sub-division of the 2nd faction that postulates that the main cause of Earth's climate change is due to variability in the energy output of the Sun. The Sun is a variable star and despite its proximity, there is still much that we do not know concerning its long-term cyclic phenomena.

It has recently been suggested ^{[1] [2]} that the Earth is not alone in experiencing enhanced global temperatures. In 2005 The Mars Global Surveyor and Odyssey missions (NASA) indicated that the Martian ice caps had been steadily shrinking for three successive summers. These observations have led Habibullo Abdussamatov^[3], head of space research at Pulkovo Observatory, to come to the conclusion that Mars too is experiencing enhanced global warming and that the culprit is the Sun. According to Abdussamatov, there is a pattern of solar fluctuations that fits the observed changes in the climate patterns of both Earth and Mars. He predicts that the solar radiation, which began to decline in the 1990's will continue to fall until 2040 and this will actually lead to dramatic cooling of Earth's climate in 15-20 years.

These observations are not supported by the anti CO² adherents, who attribute Mars' temperature increase to the Martian version of Milancovic Cycles. These are more enhanced on Mars due to its greater orbital eccentricity and by the fact that Mars does not possess a large moon like ours to dampen the effects. They say that it is 'pure coincidence' that both Earth and Mars are experiencing enhanced global temperature at this time. The debate continues.

[1] Kate Ravillous National Geographic News Feb. 28 2007 Back
[2] & Steinn Sigurosson- Global Warming on Mars Back
[3] Pulkovo Astronomical Observatory Back

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