Saturday, October 17, 2015

Looking for Aliens

Ravi Kopparapu visited Zurich some time ago and gave a very interesting seminar that summarized the latest work in extrasolar planets. Some of the things I've learned are below. Note that the title comes from the description that Ravi's daughter gave of this. The actual talk title was "Habitable Zones and the Occurrence of Potential Habitable Planets in our Galaxy".

When is a planet Habitable? 
Habitable zones [Image from Ravi's website]
 A rocky planet is potentially habitable if it contains water. In our solar system, the Earth is located close to the inner edge of our habitable zone (also known as the Goldilocks zone). Mars is located close to the outer edge.  The Moon is outside our Goldilocks zone.

Ravi (together with James Kasting from Penn State University and others) built a calculator for finding habitable zones around different kind of stars. The brighter the star the further way the planet has to be to be habitable. The coolest types of stars are M-stars or red dwarfs. They are numerous and dim, and so the habitable planets in their orbits can be closer in. Planets around M-stars are also tidally locked. This means that they rotate synchronously just like the Moon does - always keeping the same side towards Earth.

Close to the inner edge of the Habitable zone, the planet has a water dominated surface like Earth. At the inner edge, the planet is so hot that most of the water has evaporated to the atmosphere, then it is no longer habitable.  At the outer edge, the temperature is low, and no amount of carbon dioxide will warm up the planet to melt the ice. For a star like our Sun, the inner edge of the habitable zone was first found to be between 0.97 to 0.99 AU, while the outer edge is at 1.6 AU. However, this model did not include could feedback.

3D models also include clouds, which reflect some of the sunlight and allow the planet to be slightly closer to the star. The inner edge of the habitable zone for a Sun-like star shifts to around 0.93 to 0.95 AU, whereas the outer edge remains unchanged (1.6 AU). Mars is at 1.5 AU, while Venus is outside the habitable zone receiving about twice as much sunlight as Earth.

It has been known for a long time (1996) that there was ice at the Martian North Pole, and beneath the surface at the South Pole. Structures and rocks on Mars also suggested the presence of flowing water. But there was no direct proof of water flowing now on the surface of Mars until the Mars Reconnaissance Orbiter has found hydrated salts. This means that very salty water still flows occasionally on Mars. 

How many planets have been found in Habitable zones?
There are about 30some known planets in the habitable zones of other stars, and out of those about 10 are Earth-sized. The numbers are so low because current technology does not allow us to see most of them, not because the planets don't exist.

How do we estimate habitability from so far away?
Life interacts with the atmosphere. We would expect to see the same kind of gases as we observe in the Earth spectrum: Oxygen, Carbon dioxide, water vapor, ozone, methane, and dominant nitrogen. Plate tectonics is also very important.  It causes volcanism, and volcanoes are believed to be what got our planet out of the various ice-ages.

Life under-surface would not interact as easily with the atmosphere, and would be harder to detect from far away.  

Snow Ball Earth
The Earth is believed to have had many ice-ages (C-Si cycle). About 700 million years ago, our planet was a snow ball. The the ice was 1 km think and reached the Equator.  There are a number of proposed triggers for the ice age - one could be the eruption of a super-volcano like Yellowstone. The carbon dioxide is taken away from the atmosphere into the subsurface of the Earth. It is called the Carbon-Silicate cycle where the surface Silicates are converted to carbonate sediments. In time carbon dioxide accumulates in the atmosphere again and the ice melts. 

How many planets do we expect around a given star?
Every star should have at least one planet of any kind orbiting it. Planets are common. They are not an exception. They exist around every star in our galaxy.

The first Earth-like planets were found more than 20 years ago. In 1992, it was found that planets could orbit pulsars. This shifts the center of mass of the system. The pulsar then wobbles around the center of mass causing millisecond delays in the pulse arrival times. 

Twenty years ago (in 1995)  Michel Mayor and Didier Queloz (Geneva Observatory, Switzerland) found the first extrasolar planet around a Sun-like star. 51 Pegasi B became a prototype for a new class of planets - the Hot Jupiters. They are big like Jupiter, but orbit very close to their stars, which induces high surface temperatures.  These planets were first found via the radial velocity method, which measures the velocity shift in the spectral lines of the star induced by the planet's gravity.

Both the temperature and mass of 51 Pegasi B are Sun-like. It has a surface temperature of over 5500 K. The light it reflects from its Sun is in the visible spectrum, and can be detected from Earth. Recent work suggests that this planet could pioneer yet another way for finding nearby extrasolar planets.

Today some 4000+ planets were found. Most were found by NASA's Kepler mission in 2009-2013 via the transit method. When the planet crosses in front of the parent star disk, the observed brightness of the star drops by a small amount. Three transits should be observed to confirm a detection.  For our Earth, a far-away alien civilization would observe one transit per year. The mission duration of four years was chosen so that Kepler could find Earth-like planets around Sun-like stars.

 How to find Alien life?  
 Keep searching and keep an open mind...

Kepler has recently found a star with an unusual a-periodic pattern. Two hypothesis were put forward: (1) a swarm of comets that are perhaps a remnant of a kind of collision and (2) an Alien
Mega-structure that perhaps consists of solar panels used by some very advanced alien civilization.
So, have we found aliens. Well... of course, not, but more data is needed, e.g., infrared and 
radio data. This is the first reasonable candidate that could be used to develop SETI methods. It is very reasonable that some more odd balls will be found when looking for planets.

Perhaps we will look at the atmosphere of habitable planets and find life outside the solar system that way or perhaps there will be some satellites built by aliens that we will see around the stars that host habitable planets. Either way it is timely to start taking SETI research seriously.

Other news
Geoffrey Marcy who led the research team that discovered the first planetary system around a Sun-like star resigned his professorship under pressure after allegations of sexual harassment. He went from being known as the finder of new worlds and being nominated for the Nobel prize to having to quit his position at Berkeley. He was also a co-Investigator of the Kepler mission. He is thus a person who changed our understanding of science.

I did not know him, but apparently his behavior had been an "open secret". So, why put the pressure now? did he disturb important enough people with unrelated behavior? was he not productive enough (he is 61) and so it did not make sense for the university to continue to cover his apparently non-criminal indiscretions? does it makes more sense to hire someone else on his position at this point? is it a combination of factors? Or is it really about setting a zero tolerance policy to sexual harassment in universities around the world? I have given up trying to understand politics for some time, and I want to think even less on these issues now that I am applying for jobs again.

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