First Earth-Sized Planet in Habitable Zone

The last few decades have been an exciting time for the exploration of other solar systems. So many exoplanets have been found, with the total going from literally zero to thousands. First with Kepler, then Spitzer, and now TESS — the Transiting Exoplanet Survey Satellite — which can detect minute fluctuations in the light emitted by target stars as their planets transit in front of their suns. This has been described as analogous to analysing the light from a lit-up skyscraper at night and being able to detect someone shifting down their office blind by one centimetre! More on TESS here.

All the planets of interest identified by the TESS will be classified with the TOI prefix ( Transiting Exoplanet Survey Satellite Object of Interest).

TESS has already had its first find. An Earth-sized planet in the habitable zone of the star designated TOI 700, which is 100 lightyears away. Cosmic spitting distance! For the astronomically minded, this star is in the constellation of Dorado. It’s so exciting  to imagine these stars as fostering a solar system favourable to life. TOI 700 is a cool M dwarf star, also known as a red dwarf star, the coolest in the cosmic stellar sequence, the most common, and the longest-lived stars. This red dwarf has 50% of our Sun’s surface temperature, and 40% of its radius — like a cool little sister.

TOI700 d First Earth-Sized Planet in a Habitable Zone

The planet that all the excitement is about is TOI 700 d, which is the outermost of three identified planets in that system. It is estimated to be around 20% larger than Earth, with an orbital period of 37 days, receiving perhaps 86% of the energy that our Sun provides to Earth. All three planets in this system are thought to be tidally locked to their star. This means they rotate once per orbit, with one face always toward its sun and the other permanently facing away — day and night sides — much like how the Moon is tidally locked to Earth.

Based on our solar system, we are used to the idea of rocky planets existing closer to the sun, with gas giants appearing further out. In TOI 700 the closest planet to the sun (TOI 700 b) is Earth-sized and rocky, the second (TOI 700 c) is likely have a composition similar to Neptune, while the goldilocks third planet (TOI 700 d) is Earth-sized and rocky!

What makes TOI 700 d unique is that it’s the first Earth-sized exoplanet located in the habitable zone. Astronomers have found thousands of Jupiter-sized planets, many of them “hot Jupiters” that orbit very close to their star, and other rocky planets, some of which are Earth-sized, but all of which lay outside the zone where liquid water might exist on their surface.

The only hitch for TOI 700 d is that despite receiving less solar energy, it is thought to be receiving up to 35 times more extreme UV radiation, which is not so great news for developing life. Regardless, TOI 700 d is a solid candidate for a habitable world, and one in our close stellar neighbourhood.

Future work will be targeted at characterisation of the planets’ atmospheres, and if possible, their actual compositions. Given the fact that they are likely to be tidally locked, the 3D climate modellers are going to have their work cut out for them!

The likelihood that the three planets in this system might be tidally locked has really tickled my SF brain, since one of the major premises of my SF book The Tau Ceti Diversion, was that the target planet (where all the action takes place) is tidally locked to its sun.

The Tau Ceti Diversion . . . with the crew dead, and the starship’s fusion drive held back from a lethal explosion, Karic and the surviving officers finally reach a habitable planet. The last thing they expected was to find it already occupied . . .

Get The Tau Ceti Diversion here!