Living under a different sun

St Petersburg University graduate Svetlana Berdyugina, Director of the Kiepenheuer Institute for Solar Physics and a member of the International Astronomical Union, answered these questions during an open lecture.

In the mid-1990s, scientists started discovering objects outside the Solar System — exoplanets. They turned out to be so numerous that it became necessary to clearly define the very notion of a «planet». This was done in 2006 at the 26th General Assembly of the International Astronomical Union.

There are approximately 10²² stars in the Universe, about 100 billion of them being located in our galaxy alone. Some of them have already turned into black holes or neutron stars, some have not yet evolved, but the number of objects is still extremely large. How many planets are orbiting these stars and how can we detect them?

It is now accepted that the Solar System consists of eight planets and several dozen dwarf planets, including Pluto (it was expelled from the planet club because there are many objects in its orbit that have a similar size).

Compared to stars, planets are not as bright and are not easy to observe through a telescope. Therefore, other — indirect — methods are used to detect them. The first one, spectroscopy, involves measuring the speed and the orbital period of the star, which, like the planet, is rotating around the system’s centre of gravity. In the Solar System, this distance is relatively small, so this method is effective for detecting massive planets located very close to their stars. Another method is photometry. Sometimes a change in the brightness of a star signals that a certain opaque object transits (i. e. passes) against its disk and partially blocks its light. This method makes it possible to explore even the atmospheric composition of planets: it is a way to find out if the planet is habitable. There are also other methods: for instance, Svetlana Berdyugina’s research group uses polarimetry. Although, spectroscopy and photometry are considered the most reliable ones.

For almost 30 years from the beginning of the search for exoplanets, about four thousand planetary systems have been discovered. Statistics show that, as a rule, not just one but several planets (most often 5 or 6) are rotating around their stars. More than 30% of the stars have objects similar to Earth in size, within one to three Earth radiuses — it turns out that there are quite a number of small exoplanets that are theoretically suitable for the origin of life. «How do we know whether a planet is habitable? We select certain criteria based on the Earth as an example. The presence of heat, water, and a particular composition of the atmosphere and its pressure are important. Now, the presence of liquid water on the planetary surface is considered the main habitability criterion. Its existence requires the presence of an atmosphere. For example, on Mars it is too thin for a liquid substance to exist,» Svetlana Berdyugina explained. «The habitable zone depends on the brightness of the star: it is farther away from hot blue stars, and closer to cooler red stars. The best distance for planets orbiting yellow stars, such as the Sun, makes up one astronomical unit — the distance from Earth to the Sun. Venus is located closer, and the temperature on its surface reaches 500 degrees Celsius, while on Mars it seldom reaches zero. Thus, we have determined that out of 4,000 planets, only 14 or so are suitable for Earth-like life: they are small enough and are orbiting their stars at a suitable distance.»

How do we know whether a planet is habitable? We select certain criteria based on the Earth as an example. The presence of heat and water, a particular composition of the atmosphere and its pressure are also important. Now, the presence of liquid water on the planetary surface is considered the main habitability criterion. Its existence requires the presence of an atmosphere. For example, on Mars it is too thin for a liquid substance to exist.

St Petersburg University graduate Svetlana Berdyugina, Director of the Kiepenheuer Institute for Solar Physics

What would other life forms look like? Judging by Earth’s history, people are much more likely to find bacteria on another planet than a highly developed civilisation. Unicellular organisms have existed for about 4 billion years and are still the dominant life form. Bacteria can adapt to a variety of conditions. Particular extremophile species are able to live: in hot springs, where the temperatures exceed 120 degrees Celsius; in salt lakes with sodium chloride concentrations of up to 20%; and in deserts with a humidity of less than 10%. Conditions on other planets may vary, so scientists are watching closely which bacteria adapt to extreme conditions and what energy sources they use.

However, the list of candidates for life includes not only exoplanets, but also objects of the Solar System.

Although Mars looks not quite habitable now, there are suggestions that at the dawn of its history it looked like Earth, had water and an atmosphere, although it is difficult to find evidence for that at present.

St Petersburg University graduate Svetlana Berdyugina, Director of the Kiepenheuer Institute for Solar Physics

«Nevertheless, research has shown that under the surface of the planet there is a permafrost layer, 5 m to 500 m thick. «If there was life on Mars, it could have moved underground, and in areas warmed up by the sun, the bacteria could thaw and demonstrate certain activity,» Svetlana Berdyugina said.

Other candidates are ice moons of the Solar System planets: Enceladus, Europa and Titan. It was discovered that these moons contain viscous saline matter in the intermediate region between the surface layer and the denser inner core. Sometimes geysers erupt from under the icy crust, and organic molecules were found in these emissions consisting of water. Titan, a Saturn moon, is thought to be one of the most promising places to search for life. There is no water on it, but there are stable lakes and even rivers filled with liquid methane and ethane. «If we look for life there, we should by no means expect it to be the same as on Earth,» Svetlana Berdyugina commented. «Chemical reactions may occur there and the formation of complex organic matter is possible. Yet, under such conditions, life will evolve very slowly.»