The Kepler Space Telescope has discovered 20 new exoplanets orbiting faint little stars. Five of them are inside the habitable zone. That is, where there can be liquid water and life itself. The Kepler team announced this at a joint meeting of the Planetary Research Division of the American Astronomical Society and the European Planetary Congress.
New planets the size of the Earth, sometimes slightly smaller, sometimes larger, like Neptune (these are called super-earths). They are quite suitable for dwelling there even for us, without bending from excessive attraction and without flying away into space from extraordinary lightness. They revolve around very small stars - orange and red dwarfs of classes K and M. These stars are parasites that prevent scientists from observing something significant. So, in any case, Courtney Dressing, the astronomer from Caltech who presented the discovery, christened them.
They are indeed ubiquitous: up to three-quarters of the stars in the Galaxy are red dwarfs. About 250 are close, within 30 light-years from our Sun (which is huge in comparison with them, ten times more). Courtney herself, young and pretty, insists that habitable planets be searched near such dim stars. In recent years, this has become what is now called a trend or mainstream.
So, red dwarfs. Faint stars, which are less than ten percent of the solar mass in mass, and their photosphere temperature is 3500 kelvin and below, which is almost half that of the Sun. However, hypothetically, they can live for another trillion of years, which goes beyond the horizon of the most violent imagination. The entire universe began only 13.8 billion years ago. During this time, many stars were born and died, and dwarfs intend to exist hundreds of times longer. None of the physicists will undertake to predict what will happen to the world for such a long time, but if everything remains "as before", then life in M-class stars can arise with a high probability. If not already conceived.
Kepler-20f is an exoplanet orbiting the star Kepler-20 in the constellation Lyra. Mass - 0.66 Earth masses. The orbit is the fourth from the parent star. A year on the planet lasts 19 earth days
Photo: NASA / Kepler mission
In the search for alien life, earthlings' hopes alternate with disappointments. Nobody writes messages to the earthly mind from the extraterrestrial, nowhere do we see clear traces of even primitive organisms. Hope for Mars - almost stopped. Now we are hoping for Europa, the moon of Jupiter. But most of all hope, of course, is in exoplanets (planets that orbit a star that is not the Sun).
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The first exoplanet was discovered by Polish astronomer Alexander Wolschan in 1990. He calculated that one of the neutron stars has two planets larger than Earth: one 3.4 times, the other 2.8. Since then, many planets have been discovered near other stars, and today, together with candidates (not yet confirmed signals), about five thousand of them are known.
Then what's the sensation? The fact that several planets turned out to be both Earth-like in size and in the habitable zone. Such discoveries are still rare, although there is a feeling that here it is, has begun. For example, in the summer, an earth-like planet was found near the nearest star - the red dwarf Proxima Centauri. It was calculated from observations at the La Silla Observatory in Chile.
But the Kepler telescope remains the main provider of news on worlds outside the solar system. Why has he recently begun to find so many Earth-sized planets and super-earths? Roman Rafikov, professor of astrophysics at the University of Cambridge (UK) and the Institute for Advanced Study (Princeton, USA) answered this question to our journal:
- I would not say that this is a recent trend. Kepler opened them almost from the beginning of the mission, and this is already five years. He was the first, of course, to find large planets like Jupiter, which give the strongest signal as they pass across the star's disk. The transit signal from a planet like the Earth is significantly, once in 100, weaker, therefore, for such events, you need to track many transits in order to collect statistics. It took some time, but from the very beginning of the mission, Kepler gave out planets like Neptune and similar in size to Earth.
Part of the optical system of the Kepler space telescope
Photo: NASA / Kepler mission
Observations of stars with a mass less than the Sun are good because during transit, a small planet covers most of the star's disk than during transit of a star like the Sun. Namely, the relative drop in the brightness of the star is a signal during transit. Therefore, it is always easier to find even small planets there. There are special projects, for example MEarth, that specialize in just such systems.
Is there life there? The question at the current stage of research is divided into two. First: is it possible there in principle? Second: are we able to detect it?
Let's start with the first one. The habitable zone is a rather primitive concept. It is just the area around the star, within which water on the planet's surface can exist in liquid form. Not too close for the water to turn into steam, and not too far away to freeze. If there is water, biochemical reactions take place in the cells. We introduced this concept for the simple reason that we have not seen any other life except the earthly one. Therefore, we are looking for a similar one.
Red dwarfs are faint, cold stars. Their habitable zone is much closer than that of the Sun. If we lived there, the Earth would have to move inside Mercury's orbit to get enough heat. And there would be problems. The most obvious is radiation: X-rays, powerful flares. Only the atmosphere and, in the event of flares, the magnetic field can protect against this.
Another problem is the gravity of a nearby luminary. Its tidal forces can slow down the rotation of the planet in the same way as the Earth slowed down the Moon (which is why our satellite is always turned to us with one side). Then there would always be a hot day on one side of the planet, and a frozen cosmic night on the other. Such conditions, of course, do not contribute to the emergence of life, but there is an option when the planet gets into resonance with the gravity of the star and still rotates, as happened with Mercury. The third problem is stellar wind: streams of charged particles escaping from a red dwarf could simply blow the atmosphere into space over billions of years.
The planet Proxima b revolves around the star of Proxima Centauri in the habitable zone
Photo: ESA / Hubble & NASA
There are models to get around these difficulties. And since there are models, then somewhere in the Galaxy they could be realized. Especially when you consider the number of small stars and planets around them (according to modern estimates, there are dozens, if not hundreds of billions).
Let's say there is life on one of these planets, which is similar in biochemistry to earthly life. What are the signs to find it? The answer is this: first prove the presence of liquid water and an atmosphere, and then look for biomarkers, the first of which is free oxygen. The fact is that oxygen in the atmosphere can appear almost exclusively as a result of photosynthesis by living organisms. Physical and chemical processes, of course, also create it, but not in such quantities. Several conditions must be met for this gas to appear on its own. In general, if there is oxygen in the atmosphere, then the chances of habitability are greatly increased. So far, no such planets have been found. Is it possible in principle to study their atmospheres? Hence - with terrestrial telescopes and near-space observatories?
“Something, it turns out, is already possible now,” says Roman Rafikov. - For example, the recently discovered TRAPPIST-1 system contains three planets with a size of the order of the Earth, orbiting in short orbits - one and a half and two days for two inner planets - around a dwarf star. Its mass is 8%, and its radius is 11% of the solar, the luminosity is 2000 times less than that of the Sun. In this case, the star is 40 light years from us, very close.
Recently, an international team of researchers used the Hubble Space Telescope to study the atmospheres of these planets using transmission spectroscopy. In this method, observations are carried out during transit - the absorption of starlight in the planet's atmosphere is measured at wavelengths corresponding to the chemical elements in it. This is a very difficult observation because only a small fraction of the atmosphere at the planet's limb is involved. In this case, to amplify the signal, the observers waited until both inner planets - which sit in the habitable zone - passed through the star's disk at the same time. Their combined signal was measured. Nice idea.
The result showed that these planets cannot contain extended hydrogen atmospheres without clouds. But other possibilities remain - for example, a strongly cloudy atmosphere like the Venusian one or an atmosphere of water vapor. So, the scope for further research of this planetary system is huge.
In the future, the new American infrared telescope JWST (James Webb Space Telescope, it is planned to be operational in 2018) will make such observations more or less routine.
Well? We keep our fists. We wait.
Mirrors of the space telescope JWST (James Webb Space Telescope)
Photo: NASA