About

Looking at the starry sky, humans have been speculating about the existence of other worlds and possible life there for thousands of years. Our generation is the first to know that planets are really out there. Since the discovery of the first exoplanet orbiting a Sun-like stars in 1995, almost 6000 exoplanets have been detected with certainty, and many thousands of exoplanet candidates are awaiting final confirmation of their status. Statistical estimates show that there are at least as many, or perhaps even twice as many planets in our Milky Way galaxy than there are stars – hundreds of billions. Are there planets similar to our Earth? Do exoplanetary systems resemble our Solar System? Time has come to search for answers to those exciting questions.

Exoplanets cannot be easily seen in the telescope. So as we are not able to notice a faint candle next to a bright spotlight, it is usually not possible to take a photo of a planet that is close to the shining star. Most of the exoplanets have been discovered by indirect but still reliable methods. First exoplanets were found using the radial velocity method. When planets are orbiting a star, their mutual gravitational interaction causes the star to move around the barycentre of the star-planets system. The more massive the planet and the closer to the star it orbits, the more noticeable is the motion of the star. The barycentre of our Solar System, depending on the configuration of the giant planets at the given moment, could be within the Sun or just close to its outer surface. The first exoplanet at the star 51 Pegasi has a mass about half of Jupiter mass, but is so close to its host star that a full orbit takes only 4.2 days. This forces the star to move to such a remarkable extent that could be detected with the instrumentation available to astronomers in 1990s. Due to the well-known Doppler effect, spectral lines in the spectrum of a regularly moving star are periodically shifting towards bluer (shorter) and redder (longer) wavelengths. Measuring the velocity of these movements over the years resulted in a sinusoidal curve that proved existence of the exoplanet orbiting the star. By 2024, more than 1000 exoplanets have been discovered from the motions of their host stars.

The vast majority of the known exoplanets, about 4300 in 2024, however, have been discovered by other methods – transit or eclipse method. When the planet is moving over the disc of the star, it blocks out part of the star’s light and the star gets dimmer for some time. This dimming, however, is minute, mostly less than one per cent of the total light. In most cases, this can be reliably measured only with space telescopes, outside the Earth’s turbulent atmosphere. NASA’s landmark space telescope Kepler has discovered more than 3300 exoplanets during its main mission in 2009–2013 and extended mission K2 in 2014–2018, using the transit method. Observations of almost 3000 exoplanet candidates are yet to be confirmed. Since 2018, the new telescope TESS (Transiting Exoplanets Survey Satellite) is continuing the work of Kepler, with more than 570 exoplanets discovered by the end of 2024, and over 7000 candidates to be confirmed.

There are many other methods for detecting exoplanets. While both radial velocity and transit method favour bigger planets closer to their host stars, gravitational microlensing, for example, allows to find lower mass planets further away from the star, and even rogue planets without host star. And it is still possible to get direct images of exoplanets when they are far away from its host star. About 80 exoplanets have been found this way by now.

Astronomers have their methods to derive masses, radii, densities, temperatures, and other parameters of exoplanets. Of what is known so far, exoplanets can be categorized into four types:

• Gas giants – the size of Saturn or Jupiter, or much larger, masses can extend up to several hundreds of Earth’s mass. They include both hot Jupiters in close-in orbits around their stars, and cool giants further away. They form 32 per cent of known exoplanets.
• Neptune-like planets – similar in size to Neptune and Uranus, with hydrogen- or helium-dominated atmospheres. “Mini-Neptunes”, not found in our Solar System, are smaller than Neptune but larger than Earth. Neptunian planets form about 34 per cent of known exoplanets.
• Super-Earths – about 2–10 times more massive than Earth, typically rocky planets. They might or might not have atmospheres. There are no analogues in the Solar System. Super-Earths form about 30 per cent of known exoplanets.
• Terrestrial planets – Earth-sized or smaller, with masses up to 2 Earth’s mass, mostly made of rock and metal. Some could possess oceans and/or atmospheres. Only 4 per cent of known exoplanets are terrestrial.

The above statistics raises a natural question: Is our Solar System typical among planetary systems? Immediate answer turns out to be no. However, one should keep in mind selection effects. Our detection methods are biased towards bigger planets in orbits close to their star. With current precision, it is almost impossible to detect an Earth-sized planet at a distance of 1 AU (astronomical unit) from its star, as is the Earth from the Sun. Given the about 30 years time span, during which we have studied exoplanets, it would also be impossible to discover planets on Saturn-, Uranus- or Neptune-like orbits. Nevertheless, some models and simulations show that our configuration of planets might be quite rare, and perhaps only one per cent of planetary systems could be similar to the Solar System. This exciting problem needs further investigation.

Properties of planetary systems are intimately connected with the characteristics of their host stars. In order to reliably detect and characterize planets, we need to know their host stars: What are their masses, radii, temperatures? Do they possess magnetic fields? Is there solar type activity? Do planet-hosting stars differ from stars without planets? etc. Detailed studies of the spectra of exoplanet host stars will help to better understand these star-planet connections. This is one of the main scientific goals of the EXOHOST  project run at UT Tartu Observatory from 2023 to 2025. In partnership with University College London (UCL), Uppsala University (UU), and Space Research Institute of the Austrian Academy of Sciences (OEAW), this EU Twinning project will build capacities and develop excellence in the research of exoplanets and their host stars at UT Tartu Observatory.

With current and near-future knowledge about exoplanets, we are closer than ever to start answer the eternal question: Are we alone in the Universe?