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    Looking for the Earth’s twin

    What is a hot Jupiter? How can an Earth-like planet be discovered? How can it help us learn more about our planetary system? Roman Baluev, Candidate of Physics and Mathematics, Senior Research Associate in the Department of Astronomy at St Petersburg University, answers these and other questions about modern astronomy.

    Mr Baluev, could you please explain what exoplanets are? What are they like?

    Exoplanets are planets that orbit around stars other than the Sun, i.e. outside the solar system. Among the first planets to be discovered since 1995 were the so-called hot Jupiters.

    Scientists have discovered a whole class of celestial bodies that are similar in mass to our Jupiter, but are much closer to their star, at a distance of less than 0.1 astronomical unit. As a result of such a short distance, their atmospheres are heated to enormous temperatures of about 1,000 K. The Solar system has no such planets.

    An astronomical unit is a traditional unit of measurement in astronomy that amounts to the average distance from the Earth to the Sun. For example, the distance from Mercury (the closest planet to the Sun) to the Sun is about 0.3-0.4 astronomical units.

    At first, it seemed to be the dominant class of extrasolar planets as other types were very rarely found. It can be explained by the fact that hot Jupiters were much easier to detect given the accuracy of the measuring instruments that existed at the time. Later on, however, from the year 2000 or so, when the equipment became more advanced, more distant planets, including those similar to our Jupiter, have been discovered. It became clear that the class of hot Jupiters is not that numerous.

    Moreover, among the extrasolar planets, there were also discovered hot earths, which are located very close to their stars. As a result of the high temperatures, there is no life on them, nor can there be any. There are also hot Neptunes (or hot super-Earths).

    Why are scientists searching for new planets? What does it help to understand?

    For a long time, researchers have built theories about the formation of planetary systems and based them only on the data from the solar system. However, planet Earth is quite special – we and other living organisms emerged here. Apparently, this is a rare occurrence in the Universe: we know of no other such examples. One could study our planetary system in detail down to its chemical composition and the origins, but this would not answer the question of whether it is unique or whether it is a universal standard?

    The first discoveries of exoplanets provided additional statistics which were enough to develop a new theory of planet formation. Hot Jupiters, for example, shattered existing beliefs, as their origin cannot be explained by the old theories. Our planetary system has only one Jupiter at a distance of about five astronomical units from the Sun, and it has a substantial mass. The inner region of the solar system has only small planets: Mercury, Venus, the Earth and Mars. The first exoplanets that have been discovered have a mass comparable to that of Jupiter. However, they are 20 times closer to their star than the Earth is to the Sun. Their origins are unclear: located so close to a star, they simply could not have had the material to form a planet of such mass.

    Scientists have therefore developed a theory of planetary migration. It suggests that a planet is formed far away from the star but, through interaction with the protoplanetary disk, it gradually moves closer to the star and migrates towards the central regions. We can detect the planet now by observing the scattering of material from the protoplanetary disk.

    A protoplanetary disk, or proplyd, is a disk of dense gas, which subsequently forms planets, that rotates around a young star.

    This raises the question of why our Jupiter has not migrated. It can be down to the parameters of the protoplanetary disk: the amount of matter that was initially there; its viscosity; and its chemical composition. There is a whole field of research in mathematics, hydrodynamics, and even magnetic hydrodynamics to explain this.

    There has also been further development of the theory of gravitational instability in the protoplanetary disk, which has explained the presence of such planets in the central regions around the star without migration. The discovery of exoplanets gave a good impetus to this research and formed an entire branch of astronomy.

    What can astronomers learn about exoplanets staying so far away from them on Earth?

    The planets that revolve close to the star are so hot that they emit their own light in the infrared band and this light can be detected as they drift behind the star’s disk. As the planet emerges from behind the disk, it slightly contaminates the star’s light. If we look at the spectrum, we see that the planet adds its own lines which can be registered and interpreted.

    This is how we get information about the general composition of the planet’s atmosphere. This is useful as since we know the chemical composition of the gas giants in the solar system, we can tell the differences in the chemical composition of exoplanets. This has become a branch of science in itself.

    Another field involves the study of the atmospheric dynamics of planets. When a planet passes behind the disk of a star, this effect can be recorded and the asymmetry of this phenomenon in the infrared region of the spectrum can also be measured. This provides information about the scattered light surface brightness distribution of the visible disc of the planet. After all, the star heats the planet unevenly – the atmosphere is always hotter in the centre (at the equator). Moreover, the planet rotates and, due to various hydrodynamic effects, there are strong winds. The hot spot may shift and take some non-trivial forms. This is how we can get information about the hydrodynamics and thermal profile of the planet.

    As there are many types of exoplanets, they become a kind of experimental cauldron, an experimental laboratory created by nature.

    What methods are used to detect planets?

    There are several ways of detecting extrasolar planets. One of the principal methods is the radial velocity method, or Doppler spectroscopy. Earth-based telescopes enable us to observe the star ‘wobble’ as a result of gravitational disturbance from the planet. What we can see is not even the ‘wobble’ itself but variations in the star’s radial velocity. It is the speed at which the object moves away and towards the observer, which can be measured by spectroscopy. In other words, we find an exoplanet by the change in the subtle characteristics of the star’s light, or, more precisely, by the periodic shift of spectral lines due to the Doppler effect.

    The Doppler effect, named after the Austrian physicist Christian Doppler, explains the change in frequency and length of waves caused by the movement of their source and receiver in space.

    There is also the astrometric method, when scientists measure the direct oscillation of a star around the centre of mass of the planet, rather than the spectral parameters of the star’s luminescence. This is a rather exotic method because such an effect is very difficult to capture. It was for this kind of measurement that the Gaia astrometric spacecraft was launched in 2013. It has been flying for some time now and may be able to discover many new planets in the future. However, the data it has collected so far is insufficient as such precise measurements require the full amount of information from the entire expedition, and it still needs to be processed by special algorithms.

    Another method is microlensing, which makes it possible to discover planets orbiting very distant stars. From the Earth, clusters of such distant and dull stars merge to form the Milky Way. Sometimes two unrelated stars can be at different distances from the Earth but happen to align in the same line of sight for us. At this point, the closer star will use its gravity to focus the light of the background star onto the observer. At this point, the background star will have brightened for a period ranging from a few hours to several days. If there are planets rotating around the nearest lensing star, each of them will also play the role of a small lens. On the light curve, we will see the anomalies caused by these planets.

    Everything has to be right: the stars should align on the same line; and the plane of the planets’ orbits and the planets themselves should take the right position. This is a very rare and unlikely event. This method, nevertheless, was popular at the time of the OGLE project on microlensing, during which there was discovered a considerable number of planets in our galaxy. However, this method had an important drawback as microlensing happens only once for each object.

    The OGLE (Optical Gravitational Lensing Experiment) is a Polish-American project to study dark matter using the method of gravitational microlensing.
    One of the current methods of detecting exoplanets, which competes with the radial velocity method, is the transit method. It uses photometry and is primarily aimed at the planets orbiting close to the star. At such a distance, the planet’s plane of rotation is likely to pass through the Earth and we will periodically see the celestial body projected onto the star’s disk. The planet in this case remains invisible to us, but we do observe that it slightly dims the star’s light by about 1% in recurring periods. To spot this, we need precise photometry, which is simpler than the radial velocity method. The Doppler method requires special highprecision spectrographs. In the case of the transit method, such precision is not required.

    The transit method, however, has another drawback: the planet’s orbit has to be oriented towards the Earth for the transits to occur periodically. If its orbit is flat, the transit will not be detected. The probability of such a plane orientation is quite low. If a planet is as far away from a star as the Earth is from the Sun, the probability of detecting it is very small. It increases if the planet orbits close to the star, but is still low.

    What method do you use in your research?

    The transit method has a spin-off, namely the transit-timing variation. Suppose there is a planet orbiting a star, and due to a certain plane of its orbit it periodically passes in front of the disk of that star. If there is one planet, the transit repeats with the planet’s orbital period.

    However, if there is another planet that remains invisible, it will gravitationally affect the first planet and perturb its motion, thus disturbing the strict periodicity. So, one transit event will be a little delayed or ahead of the projected moment. Such deviations might suggest that there is another object in the system. Celestial mechanics can tell us a lot about an object and even help to calculate and construct its orbit.

    Such deviations in timing can also occur because of the tidal interaction of the planet with the star. Over time, a planet loses energy and spirals slowly towards the star due to the small distance between them. The tidal force causes them to affect each other in the same way that the Moon causes the Earth’s tides. The planet is flattened and this deformation causes a continuous change of direction so the planet always faces the star with one side. Due to this effect, there is a loss of energy in the planet’s core. This means that the orbital velocity of the celestial body gradually increases. According to Kepler’s law, the closer an object to the star is, the faster its companion should move. It is a microscopic effect: in the case of the Earth, for example, the accumulated deviation in timing turns out to be only about a couple of minutes over a 10-year observation period.

    There are two such planets known today: WASP-12 and WASP-4. The latter is being observed in South America by amateur astronomers at our request as part of the EXPANSION project, which I will talk about a little later. The study was carried out in parallel with another international team and they happened to publish the results first as they had observed the accelerating moments of the transits. We were more cautious and noted some complications in interpreting the data.

    The observed effect could have been the result of systematic errors, in particular the impact of stellar spots. If a star has homogeneous brightness over its entire disk, the transit will look beautifully smooth, just like in the textbook. Stars, however, almost always have spots and it may happen that a planet will take a ‘splash’ over the top of this spot during its passage. Then the photometric curve would show an anomaly, which would distort the result. In the end, we did confirm the timing acceleration effect, but the amplitude of the systematic acceleration was half what the second scientific team had claimed.

    How do astronomers acquire data on exoplanets?

    A colleague of mine, Evgenii Sokov, has organised an international network of telescopes among amateur astronomers, which also includes professional observatories. The network is made up of several dozen telescopes that conduct regular observations of the transits of various exoplanets across the sky. There are now just over 20 such planets, and WASP-4 was one of them. These planets have long been known and described, and we continue to accumulate data on their timings, thanks to the project.

    This project sprang from the Czech Exoplanet Transit Database. For some time, observations of varying quality have been accumulated in this database, but most of them are not of very high quality as they were taken by amateurs. Such data should be carefully selected and include only the objects whose data quality is more or less adequate. On the basis of this database Evgenii Sokov has founded the EXPANSION project and brought together people who are willing to conduct observations of exoplanets on a regular basis.

    We also cooperate with the Special Astrophysical Observatory of the Russian Academy of Sciences. They have recently commissioned a new spectrograph with the level of precision that enables observations via the radial velocity method.

    Is there any chance of finding an Earth twin?

    Astronomers around the world would certainly wish to discover such a planet. This is the cutting edge of exoplanet research and the most intensive studies are being conducted in this field. However, the task is not easy: you need to find a planet with the same mass and at the same distance from the star as the Earth. There is no point in finding a white-hot Earth where no life can exist.

    A full twin to the Earth has not yet been found. However, similar planets have been discovered near low-mass red dwarfs. Due to their low mass, these stars are more sensitive to planetary disturbances, making it easier to discover lower-mass exoplanets near them. Red dwarfs also have a life zone closer to the star because they produce a fainter light than the Sun. Their exoplanets can orbit closer to the star without getting as hot as hot Jupiters.

    The habitable zone or life zone is the area around a star with the most favourable conditions for Earth-like life.

    Looking for a complete analogue of the Earth requires a high precision spectrograph with a radial velocity measurement accuracy of 10 centimetres per second. The best spectrograph available today only allows an accuracy of 30 centimetres per second. The search for twin Earths is therefore a great challenge for engineers in many ways. High precision instruments need ultra-high stability. To achieve this, they are installed in a special protective case that maintains constant pressure and temperature.

    High precision instruments are not enough. It is important to remember about spots and other unstable phenomena, such as flares, granulation and so on, in the photosphere of a star. Roughly speaking, the surface of a star is turbulent and this causes additional noise and distortion in the measurable radiant velocity. As a physical object, a star’s radial velocity doesn’t change. However, the problem is that it is not measured directly – we use a spectrograph based on the Doppler effect. The spectra of a star reflect its unstable outer envelope. This instability varies by one metre per second.

    In short, to minimise the natural astrophysical noise of a star, it is necessary to create special algorithms that will process and filter it. This is the only way to achieve the accuracy needed to discover an Earth-like planet. No matter how difficult it is, I think it will happen sooner or later.