Give birth to worlds - exoplanets

When the Kepler space telescope ended its mission, which began in March 2013, in October 2009 due to failure, scientists loudly expressed their satisfaction with its achievements. However, it turned out that the planet-hunting adventure didn't end there. Not only because Kepler is broadcasting again, but because of the many new ways to detect them.

The telescope, circling in a heliocentric orbit, that is, revolving around the Sun like the Earth, had to explore the brightness of 150 1. Sun-like stars fringing the constellations Cygnus and Lute (XNUMX). For several years, he consistently sent signals about subsequent discoveries. esoplanet.

Among the first were Kepler-4b, 5b, 6b, 7b and 8b. Each of them revolves around its star at a small distance from it (the year on these planets is only a few days). All of them are huge - about four times the size of the Earth (Kepler-4b has a radius of 1,5 Jupiter radii). The first discoveries confirmed previous assumptions about the community of planets in space.

However, these were unearthly objects, so there was no sensation. In September 2011, researchers at the SETI Institute in California, analyzing data from Kepler, discovered the Kepler-16 system (200 light-years from Earth).

It consists of two stars - a red dwarf with a mass of 0,2 masses of our Sun and an orange dwarf (with a mass of 0,62 masses of our Sun), around which a Jupiter-type giant revolves.

1. Exoplanet search area at the moment

It was the first binary star system with a planet discovered by so-called transits. It lies in the fact that when the satellite planet of a star moves in front of its disk, it causes its periodic dimming (takes away part of its radiation). Analyzing the intensity and duration of this phenomenon, you can calculate the radius of the planet and the period of its application.

In December 2011, news came that Kepler had discovered two planets, one the size of Earth and a smaller one in the Kepler-20 system located at Lut, 950 light-years away, with five planets, of which Kepler-20e is smaller from Venus, the radius of Kepler-20f is almost equal to the radius of the Earth. Unfortunately, none of these planets lie in the so-called habitable zone, that is, the region around the stars (not too close to them), suitable for life, where liquid water can exist.

This area is often referred to as the "Goldilocks Zone", referring to the motives of a popular children's fairy tale. It's "just right for life". However, Kepler-20e and 20f are not in this e-zone, too close to their star, and their surface temperature is 500-760°C. As you can see, the tension was mounting, but we were still ahead of the climax of the cosmic spectacle that the telescope had given us.

After the first two years of his work, researchers recognized that although Kepler did discover new planets, they were most often bodies several times the size of the Earth and usually orbiting very close to their parent stars. In 2013, Kepler finally found what made the hearts of other Earth seekers beat faster.

In the Kepler-62 system, which consists of an orange dwarf (70 percent of the mass of our Sun) and five planets, two - Kepler-62e and Kepler-62f - are only slightly more massive than Earth and lie in the habitable zone. 62f has a radius of 1,4 Earth radii, is a rocky planet and orbits its star in 267 days. A year ago, in turn, fantasies about the appearance of the planet Kepler-186f in the constellation Cygnus, the next on the list of "second Earths", flashed through the world media.

It owes its fame to its size - very similar to Earth - and to what is in the life zone. Kepler-186f is one of five Earth-sized planets in a star system nearly 500 light-years from the Sun. But only it lies in a residential area. The rest are too close to the star.

Benefits of transit

The first extrasolar planet was discovered in the early 90s. The team of pioneers included the Polish radio astronomer Alexander Wolszan. It revolves around a pulsar, that is, a neutron star that emits pulses of radiation with unusual regularity. When a pulsar has a planetary companion, the regularity of the pulses is disrupted. There are not many pulsars in space, so it is difficult to consider them as the basis for the search. esoplanet.

2. Methods for detecting exoplanets in historical terms.

Therefore, other methods have been developed, such as Doppler spectroscopy, which studies the radial velocities of stars. This speed fluctuates when the star has a companion in orbit (in this case, a planet) that affects it under the influence of gravity.

Then the star moves away from the observer a little and approaches him. When it moves away, the velocity is positive (the spectral lines of the star are shifted towards longer wavelengths).

And vice versa when the star approaches. Until now, radial velocity studies have been most often used in observations from the Earth.

Minor terrestrial planets are unlikely to be found this way. Therefore, alien hunters most often use the aforementioned method called transit. The Kepler space observatory is based at its base.

The telescope is equipped photometer 95 cm (a kind of telescope), with a matrix consisting of forty-four very sensitive CCD circuits, that is, used in modern digital cameras. The total resolution of the Kepler telescope is 95 megapixels. There are other methods of searching for alien planets - the technique of gravitational microlensing, astrometry and direct observation, but at present the transit method is dominant. About 95 percent are detected thanks to him esoplanet (2)

Size isn't the only thing that matters

It is estimated that 17 percent of the stars in our galaxy have a planet or terrestrial planets, i.e. with a mass comparable to the mass of the Earth (3). Although most of them follow narrow orbits similar to those of Mercury, they revolve around different stars, some of which are much smaller than the Sun. According to scientists, about half of all stars in the Galaxy revolve near planets of different sizes, which means they are comparable to the Earth and larger.

It turns out that at least two-thirds of the stars in the Milky Way have planets. Their number is currently estimated at about 400 billion. Assuming that planets do not usually exist in isolation, our estimates turn into hundreds of billions of planets. There are at least 17 billion Earth-sized planets in our galaxy.

The number was announced in 2013 by scientists at the Astrophysical Center at Harvard University, based primarily on observations made with the Kepler space telescope. Of course, these data should not be taken to mean that each of these several billion planets has favorable conditions for life. One size is not enough. It is also important to move away from the star around which the planet revolves (4).

In November 2013, the Ames Research Center in California hosted a science conference—NASA's facility managing the Kepler mission—to sum up the scientific advances resulting from the telescope's work. According to the scientists who spoke there, every fifth star comparable to the Sun (that is, yellow dwarfs) “should” have at least one terrestrial planet (that is, of comparable size, mass and temperature). He could be born on it and survive in life as we know it from Earth.

However, it must be acknowledged that for the most part egzoplanety extremely varied. They come in different sizes and compositions. Some of them don't fit everything we know about the planets from observations of our solar system. Some are blacker than coal, others bathe in a sea of ​​lava, others drown in the oceans or lash with diamond rains.

4. The zone of life in planetary systems depending on the temperature of the star

K2 or reincarnation

Kepler The first reaction wheel of the Kepler telescope stopped working in July 2012. There were three more that allowed the probe to navigate in space. It seemed that Kepler would be able to continue observing until 2016. Unfortunately, in May 2013, the second wheel failed. Attempts were made to use corrective thrusters to position the observatory, but the fuel quickly ran out.

In mid-October 2013, NASA announced that Kepler would no longer search for planets. And still since May 2014, a new mission has been carried out by an outstanding exoplanet hunter, referred to by NASA as K2. (5). This was made possible through the use of slightly less traditional techniques. Since the telescope would not be able to operate with two working reaction wheels (you need three), NASA scientists decided to use solar radiation pressure as a "virtual reaction wheel".

This method proved successful in controlling the telescope. As part of the K2 mission, observations have already been made of tens of thousands of stars. In December 2014, it was reported that in the star system KOI-3158, located 117 light years from Earth, there are not only five so-called Earth-like planets, but they are both egzoplanety the oldest one found.

The discovery was made by a team of astronomers and astrophysicists led by Thiago Campante of the University of Birmingham, who is analyzing observations from K2. The star system KOI-3158, which is actually part of a triple star system with two M dwarfs, is estimated to be over 11 billion years old. The planetary system in the constellation Lyra, however, is not like ours because all five planets orbit a star closer than Mercury in less than ten days.

K2 brings new discoveries almost every month, because in early 2015 we learned about the existence of the planetary system EPIC 201367065, in which three small super-Earth exoplanets were discovered. In total, in the first months of 2015, sixteen esoplanet, fifteen of them were with K2.

Most of these facilities are relatively small. The current list of the Encyclopedia of Extrasolar Planets (March 2015) includes 1906 confirmed exoplanets. The other lists differ slightly in values, but none of them have exceeded 2000 extrasolar planets yet.

5. Visualization of search areas in the K2 mission - the Kepler telescope

Possible from Earth

More and more to be discovered esoplanet ground instruments are used. Even new observatories are being built, the main purpose of which is to detect and collect data on planets orbiting other stars. That's why the Minerva (Miniature Exoplanet Radial Velocity Array) was created in Arizona in Arizona. It is also an example of a new low-cost approach to finding planets.

6. Small telescopes HATNet compared to a large observatory

Instead of huge telescopes worth millions, the Minerva project uses smaller telescopes available on the market. Four such devices with a diameter of 70 cm and a length of 2,5 m, manufactured by PlaneWave, cost PLN 200 each. hole. to everyone and available to interested hobbyists who can afford the expense.

At the Minerva Observatory, the light from all four is directed to a spectrograph, which splits it into different wavelengths. This allows you to observe small changes in the movement of stars relative to the Earth.

This, in turn, points to bodies orbiting these distant suns and allows their masses to be calculated. Minerva also makes it possible to measure small changes in brightness as objects move between the disk of a star and the Earth. It focuses on 164 nearby points in space that were chosen for their potential "planetary efficiency".

The idea of ​​using relatively small and inexpensive telescopes to catch exoplanets is already being used in many places around the world. For example, the HATNet (6) telescope network, created by Gaspar Bakos of Princeton University ten years ago, uses devices as small as 10 cm in diameter and has published publications on 56 discovered planets.

The sister network HATSouth operates 24 telescopes located in the southern hemisphere in Australia, Namibia and Chile. The creation of a distributed network and its coordination allow more accurate observation of planetary transits. The Automated Planet Finder has been working atop a California mountain since early 2014. He has already located two possible planetary systems.

The collected data will form the basis for further observations using the Transiting Exoplanet Survey Satellite (TESS), which is scheduled to launch into space in 2017. Greg Laughlin of the University of California at Santa Cruz, who is involved in data collection, including the observatory, recently told NewScientist that talking about "hunting" egzoplanetybut rather about “collecting” them.

Sniff out signs of life

The transit method says little about the planet itself. You can use it to determine its size and distance from the star. A method of measuring radial velocity can help determine its mass. The combination of the two methods makes it possible to calculate the density. Can you take a closer look exoplanet? It turns out it is. NASA has already seen better planets such as Kepler-7b, for which the Kepler and Spitzer telescopes have mapped clouds in the atmosphere.

7. First image from Gemini Planet Imager - Beta Pictoris b system

It turned out that this planet is too hot for life forms known to us. It is hot there from 816 to 982°C. However, the very fact of such a detailed description of it is a big step forward, given that we are talking about a world that is a hundred light years away from us.

Adaptive optics, which is used in astronomy to eliminate disturbances caused by atmospheric vibrations, will also come in handy.

Its use is to control the telescope with a computer in order to avoid local distortions of the mirror (on the order of a few micrometers), thereby correcting errors in the resulting image.

This is how the Gemini Planet Imager (GPI) located in Chile works. The tool was first launched in November 2013. GPI uses infrared detectors, and they are powerful enough to detect the light spectrum of objects as dark and distant as they are. egzoplanety.

Thanks to this, it will be possible to learn more about their composition. The planet Beta Pictoris b (7) was chosen as one of the first observation targets. In this case, the GPI works like a solar coronagraph, that is, it covers the disk of a distant star to show the brightness of a nearby planet.

It was with the help of adaptive optics that the Palomar Observatory in San Diego, California discovered planets in the quadruple star system 30 Ari in the constellation Aries (8), 136 light-years from Earth.

For reference, the first planet in such an exotic four-star system, KIC 4862625, was discovered in 2013 by amateur astronomers analyzing Kepler data.

This just shows that this is not an entirely rare phenomenon, and quadruple systems make up a fifth of the known sun-like stars (9). The key to observing "signs of life" is the light from a star orbiting the planet.

Light passing through the atmosphere egzoplanety leaves a specific trace that can be measured from the Earth by spectroscopic methods, i.e. analysis of radiation emitted, absorbed or scattered by a physical object. A similar approach can be used to study the surfaces of exoplanets. However, there is one condition. The surface of the planet must absorb or scatter light sufficiently.

Evaporating planets, that is, planets whose upper layers float in a large dust cloud, are good candidates. As it turns out, we can also recognize things like cloudiness on a planet from a distance. The existence of a dense veil of clouds around esoplanet GJ 436b and GJ 1214b were deduced from spectroscopic analysis of light from their parent stars.

Both planets are classified as "super-Earths". GJ 436b is located 36 light years from Earth in the constellation Leo. GJ 1214b is in the constellation Ophiuchus, 40 light years away. The first is similar in size to Neptune, but much closer to its star than the planet known from the solar system. The second is smaller than Neptune, but much larger than Earth.

9. Vision of the world in a triple star system

European things will happen

The Space Agency (ESA) is currently working on a satellite whose task is to accurately characterize and learn about the structures already known esoplanet (CHEOPS). The launch of this mission is announced for 2017. NASA, in turn, wants to send the aforementioned TESS satellite into space in the same year, which will be focused primarily on searching for terrestrial planets, about 500 XNUMX. stars closest to us.

10. One of the visualizations of the James Webb telescope

The plan is to discover at least three hundred "second Earth" planets. Both of these missions are based on the transit method. In February 2014, the European Space Agency approved the PLATO mission.

According to the current plan, it should start in 2024 and look for rocky planets with water content. These observations may allow - in the same way that Kepler's data was used for this - to also look for exomoons.

The sensitivity of PLATO will be comparable to that of the Kepler telescope. The European ESA developed the Darwin program many years ago. NASA had a similar "planet finder" - TPF (Terrestrial Planet Finder).

The aim of both projects was to study Earth-like planets in terms of the presence of gases in the atmosphere, which signal favorable conditions for life. Both included the bold ideas of creating a network of space telescopes collaborating in the search for Earthlike ones. esoplanet.

However, ten years ago these technologies were not sufficiently developed, and the programs were curtailed. Fortunately, not everything went to waste. Enriched by the experience of NASA and ESA, they are currently working together on the James Webb Space Telescope, due to be launched into space in 2018 (10). Thanks to a large 6,5-meter mirror, it will be possible to study the atmospheres of large planets.

This will allow astronomers to detect chemical traces of oxygen and methane. We will already receive specific information about the atmospheres of exoplanets - the next step in refining knowledge about these distant worlds. Various teams are working at NASA on further research in this area. One such lesser known and still in its early stages is Starshade.

We are talking about shading the light of a star with something like an umbrella, on the outskirts of which planets could be observed. By analyzing the wavelengths, it would be possible to determine the components of their atmosphere. NASA will evaluate the project this year or next and decide if the mission is worth it. If the Starshade mission is launched, it will happen in 2022.

Exo-moons wait in the wings esoplanet Astronomers are also looking for exo-moons — ones that have an even better chance of having life than the planets they orbit. Traditionally, astronomers are interested in life zones (ecospheres) in star systems. However, exomoons complicate this simple picture. Solar radiation is not the only way to keep warm.

The home planet can give the moon additional energy through gravitational compression and stretching, a process called tidal heating. This allows the Moon outside its life zone to maintain as much heat as is necessary to sustain liquid water. This phenomenon at least doubles the size of the life zone. The moon Europa, for example, experiences Jupiter's strong gravitational field.

This keeps the icy oceans fluid. One of the detection methods esoplanet Microlensing is based on the phenomenon of light bending under the influence of gravity when an object passes between us and a distant star. Its gravity focuses light like a lens, and the star's brightness periodically increases. During recent observations with the MOA-II telescope in New Zealand, astronomers noticed a sharp increase in the star's brightness, followed an hour later by another, weaker increase.

Consequently, a large object with an accompanying smaller one must have passed in front of the stellar disk. The measured mass ratio of both objects was approximately 2000:1. According to some astronomers, the accompanying Moon may mark its presence by causing the planet to wobble slightly. In practice, this means that the transit of the planet may be delayed or accelerated beyond what is expected.

Perhaps there is also an additional decrease in the brightness of the star due to the Moon itself? As part of the Hunt for Exmoons with Kepler (HEK) program, a team of researchers selected 250 exoplanets to study satellites. The work is slow and tedious to analyze each planet in turn using computer programs that could detect deviations and additional changes in brightness.

The scientists analyzed seventeen planets over two years. So the hunt for the first confirmed exomoon could take some time. However, when an appropriate method is finally developed, their numbers may quickly outnumber the planets. After all, in our solar system there are only eight planets and as many as 145 moons (and 28 more await confirmation). As you can see, this multiplies the number of worlds from hundreds of billions to hundreds more.