Bengaluru: NASA’s latest planet-hunting telescope, the Transiting Exoplanet Survey Satellite (TESS), has discovered three new exoplanets orbiting the star TOI-270, just 73 light years from the Sun. One of them is a rocky planet bigger than the Earth, while two others are gaseous bodies half the size of Neptune.
The planets are unusual for the kind of star they orbit and the location they are in. They can potentially be the missing link in planetary-growth between smaller rocky planets, like the first four in our solar system, and the larger gas and ice giants, as the last four.
Studying these bodies can help researchers understand the evolution of planets in the Earth’s solar system and if terrestrial and gaseous planetary bodies follow the same formation process.
The results were published in Nature Astronomy this week.
Types of exoplanets and discovery methods
Exoplanets are classified according to their size and mass — with Earth, Neptune and Jupiter as points of reference. For scale, Neptune’s diameter is four times that of Earth and that of Jupiter is about three times that of Neptune.
A sub-Earth exoplanet is less massive and smaller than the Earth, while Earth-like or Earth-sized exoplanets are self explanatory. Super-Earth exoplanets are larger than the Earth and up to 10 times its mass. Larger and more massive than super-Earths are sub-Neptunes, which are still smaller than the Neptune.
Hot Jupiters are gas giant exoplanets that are similar in mass and size to the Jupiter but orbit very close to their host star, while super-Jupiters are larger than the Jupiter.
Launched in April 2018, TESS is an orbiting telescope that identifies exoplanets closest to the Earth.
Stars blink when exoplanets pass in front of them and this is captured by the untra-sensitive TESS to calculate the size of exoplanets. The larger a planet, the greater is the dip in a star’s brightness. This is called the ‘transit method’ of finding planets outside our solar system.
NASA’s now-defunct Kepler telescope had performed similar functions and confirmed the presence of 2,702 exoplanets. However, Kepler could discover only a handful of planets that are between 1.5 and 2 times the Earth’s radius. The newly discovered exoplanets are 1.25 times, 2.13 times and 2.42 times the radius of the Earth.
Such a finding is very rare. Comparing the smaller rocky one to the larger gaseous exoplanets can point to whether they have evolved and formed in the same way, extrapolating the process to our own solar system.
Super-Earths and sub-Neptunes are a mystery to humans, having no example in our own solar system to study. Understanding their formation and habitability has eluded scientists so far.
But the TOI-270 system is very close to Earth, in astronomical terms. The planets also orbit close to their host star too, circuiting in 3.36, 5.66, and 11.38 days.
The orbital resonance (a gravitational phenomenon in which two bodies that are orbiting around a parent body are in a specific pattern of orbital duration) of these planets are 5:3 and 2:1. This means that for every five orbits the inner planet takes, the second one does three, and for every two orbits the second one does, the outer one does one.
These orbital resonances exist in our solar system too. Neptune and Pluto orbit in a 3:2 resonance, while the Jupiter moon pairs Io-Europa and Europa-Ganymede orbit in a 2:1 resonance.
Using these resonances, and considering the TIO-270 system’s proximity to Earth, another method of exoplanet detection can be employed to understand masses of exoplanets.
The radial velocity system, for example, utilises the effect of a star and a planet’s gravitational pull on each other. Two bodies exert gravity on each other and rotate around a common ‘barycentre’ – a point in space between them. For systems where one object is disproportionately massive than the other, like the Earth and the Moon, the barycentre exists within the larger body. For two bodies of comparable masses, like Pluto and its moon Charon, the point lies outside.
In both scenarios mentioned above, the satellite’s ‘tug’ on a host can be observed in the latter’s movements. Scientists can use this method to further analyse the three new planets and understand their composition.
Masses of planets can also be measured by a method called transit timing variations (TTV). In closely-packed planetary systems, the planets also tug at each other, causing one planet to move faster and another a little slower. This leads to measurable orbital period changes which can be used to calculate masses of planets.
“It’s quite exceptional to be able to measure masses by two different methods,” said Maximilian Günther , the lead author of the study.
TOI-270’s proximity also means that scientists can also employ spectroscopic methods to understand its atmosphere. This is beneficial since its outermost planet is just on the edge of being habitable.
“The equilibrium temperature of the outer planet (340 ± 14 K) lies within the survivable range for extremophile organisms (those that thrive in physically or geochemically extreme conditions which are detrimental to most life on Earth),” write authors in their paper.
Finally, the host star of the TIO-270 system is a red dwarf. Red dwarfs make up the largest number of stars in the galaxy and our closest star, Proxima Centauri is one. They are older than most stars and their smaller size makes them burn at lower energy levels for longer periods of time. They are dimmer and cooler than the sun, but they are not completely inactive.
Red dwarfs often exhibit solar storms and intense flares just the way the Sun does. But the TOI-270 star is very quiet, giving off a steady glow with no noticeable fluctuation. This drastically increases chances of habitability due to stable environmental conditions. The outer gas planet is thought to have a habitable upper atmosphere but a densely packed lower one, making the planet extremely hot.
“It’s a perfect laboratory,” said Günther of the TOI-270 system.