September’s Talk

Patrick Irwin is Professor of Planetary Physics at RAL, and has a particular interest in gas like and exoplanets and the atmospheres thereof. He was involved with some of the Venus Express, Rosetta and Cassini equipment. He started off as a member of Bath AS and now asks how we can detect life on exoplanets.

Before 1995 the only planets we knew were those in our solar system, i.e., the terrestrial ones and the gas giants. Then along came the discovery of 51 Pegasi b, which I found out is a hot Jupiter with a two day orbit. We discovered it by the radial velocity of the spectral lines on the star; the planet is making its star wobble so the spectral lines keep shifting as the pair orbit around the system’s centre of gravity. (Hot Jupiters are large gas giants orbiting very close to their parent stars. If you’ve attended some of our previous talks on exoplanets you will know that these planets’ orbits are decreasing and sending the planets into certain doom.)

Transit spotting is a stock method for detecting exoplanets because they are so much fainter than their parent star. Unfortunately the orbital orientation can’t be ascertained. That can only be done if we can actually see the planet transiting the star. If you get a dip in the star’s light then you can assume a planet is transiting, and if the orbit is just off horizontal to us then you get a mini dip when the planet going behind its star. I loved his demonstration with a golf ball and a football stuck on either end of a stick hanging from a string. He also showed us the best picture I’ve ever seen of Venus in transit.

There are also two ground based observatories that can do direct imaging of exoplanets by using a coronagraph, which is a way of blocking out the parent star’s light so the exoplanet becomes easier to see. Gravitational lensing can also reveal a double image of a star passing behind a darker star. When the two stars are in a line of sight the gravity of the front star bends the light coming from the star behind it. If the star has a planet you get a double spike in the two light curves of the star. Of course this is just a one off, line of sight, event. Mind you, 25 exoplanets have been observed using this microlensing effect, as it is called. But, really that’s a tiny proportion of the 2950 exoplanets that have already been confirmed, and the unconfirmed 2505 possibilities from the Kepler data. Prof Irwin concludes that almost any population II stars (metal rich, second generation stars) have planets, including hot Jupiters and Earth sized.

As regards finding planets that may harbour life, we need to look at exoplanets in the ‘Goldilocks zone’, as it is called, where liquid water can exist on the surface. This depends on the planet’s distance from its star and the temperature of the star. Mind you in theory it seems that Venus and Mars are supposed to be suitable in our Solar system.

Transit observations give us the most data. Prof Irwin did not go into detail about the equipment involved, but then we have had previous speakers talk about exoplanet searches. Kepler is still in space in an Earth-trailing orbit, although its steering has been compromised. It had been observing a patch of sky in Cygnus and in spite of only managing three years of its intended ten years of data gathering it is still providing information. It is a Schmidt plate camera with a 55 inch primary.

Pictures can be taken at different wavelengths. Red gives shallow readings, green and blue go in deeper, so you can get better absorption readings of the planet’s atmosphere. Different gases have different absorption signatures, such as methane, ozone, carbon dioxide.

An exoplanet can also exhibit a change in light as its phase changes, so it is higher as the star moves behind it. Even thermal emissions can be detected.

Our Earth’s atmosphere does not have a uniform depth; the ozone holes and oxygen are not permanent and need to be constantly replenished. They are clear signs of life. Intriguingly, the temperature of the carbon dioxide in our upper atmosphere is warmer over Antarctica than elsewhere.

Prof Irwin gave us a review of the Cassini-Huyghens mission and the equipment he was involved with. It is still the largest ever interplanetary craft that we have launched. He was involved with CIRS (Composite Infra-Red Spectrometer), which operated at long IR wavelengths and had to be cooled to 80 kelvin. It was pointed at Saturn and had a black shield which radiated heat into space. He brought in a model of the instrument, with its tiny germanium lens and hollow titanium legs, also tiny and quite fragile. It had to be flight tested so that it could survive two years of travel with its protective cover on before it could be deployed. He said the shake test involved standing it on a large woofer.

In July of 2004 Cassini-Huyghens was sent on lots of elliptical orbits round Saturn and observed lots of swirly bits along belt zone boundaries. He got quite excited about them and still does not know why these swirly bits exist. He had also noted how the rings appear bent as they go behind Saturn but concludes Saturn has an atmosphere which bends their light.

He was also involved with VIMS (Visible and Infra-red Mapping Spectrometer), which looked deep into Titan’s atmosphere, wavelength around five microns. It also revealed lots of structure in Saturn’s atmosphere, even more than in Jupiter. The rings’ reflections on the surface and backlighting from the rings is exquisite. Internal heating is detected.

Titan’s clouds have a visible orange smog but near IR shows deeper methane clouds and a methane rain (hydrological) cycle.
CIRS took spectrum readings of Titan, and took temperature readings at different heights and latitudes.

The hexagon shaped vortex on the north pole of Saturn was actually detected by Voyager two, but the image was confusing at the time. It is very clear at five microns and backlit. A big storm in June 2011 recorded the hottest temperature at the uppermost part of Saturn, visible in a colourful swirl.
Prof Irwin’s final thoughts were on the future of ground based telescopes as well as the space based scopes such as the JWST.

Clear skies.

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