An Earth-Size Exoplanet In Its Star's "Goldilocks" Zone

 Ever since the first exoplanet was discovered a generation ago, astronomers have learned to expect the unexpected. For over twenty years, a fantastic treasure trove of weird Wonder Worlds have been discovered. Indeed, some of these very alien planets, in orbit around stars beyond our Sun, are so bizarre that astronomers never thought anything like them could really exist in the Cosmos--that is, until they were discovered. Strange distant worlds aside, the Holy Grail of planet-hunting astronomers has long been to find worlds more like home. In January 2020, astronomers announced the discovery of just such a long-sought world--the first to be found by NASA's Transiting Exoplanet Survey Satellite (TESS). The distant Earth-size planet is comfortably located in its star's habitable zone The habitable zone of a star is that "Goldilocks" range of distances where conditions are not too hot, not too cold, but "just right" for liquid water to pool on the surface. Where liquid water exists, life as we know it may also exist.

The Earth-like world, named TOI-700 d, orbits a small red dwarf star dubbed TOI-700, that is only 101.4 light-years away in the Dorado constellation. That star is the brightest known stellar host of a transiting habitable zone, Earth-size world. The acronym "TOI" refers to stars and exoplanets studied by TESS. The red dwarf star, TOI-700, is of spectral class M, and it is 40% the mass, 40% the radius, and 50% of the temperature of our Sun. The bright star also displays low levels of stellar activity. Red dwarf stars are the smallest--as well as the most abundant--true nuclear-fusing stars in our Milky Way Galaxy. Because they are so small and cool, they can "live" for trillions of years. In contrast, our somewhat larger Sun can only "live" for 10 billion years. Very massive stars can only "live" for millions of years because their intense heat causes them to burn their supply of nuclear fuel more rapidly than their smaller stellar kin. The bigger the star, the shorter its "life."

The first scientific discovery of an exoplanet was made in 1988. After that, the first validated detection was made in 1992, with the discovery of several terrestrial-mass planets in orbit around the pulsar PSR B1257+12. A pulsar is the remains of a massive star that has ended its "life" in a core-collapse (Type II) supernova blast. Pulsars are young neutron stars that are born spinning rapidly with a regularity frequently likened to a lighthouse beacon on Earth. They are city-sized objects that are so dense that one teaspoon full of their material can weigh as much as a thundering herd of wild horses. In effect, these baby neutron stars are one enormous atomic nucleus. A pulsar was one of the last stellar objects that astronomers thought would play host to a family of planets--that is, until they were discovered. The pulsar planets were the first of a long series of oddball exoplanet discoveries. They are hostile small worlds that are mercilessly showered with their parent-pulsar's deadly beams of radiation.

The first confirmation of an exoplanet, orbiting a "normal" hydrogen-burning star like our Sun, was made in 1995. This new discovery also proved to be a surprising oddball--a giant planet circling fast and close to its searing-hot stellar parent. The planet, 51 Pegasi b, is in a roasting 4-day orbit around its star, 51 Pegasi. As it turned out, this large planetary "roaster" was the first of a new and unforseen class of exoplanet--hot Jupiters--to be discovered. Since 51 Peg b's discovery, many others of its bizarre kind have been observed in orbit around stars beyond our Sun.

Some exoplanets have been imaged directly by telescopes. However, the vast majority have been discovered via indirect methods, such as the transit method, whereby a planet is found floating in front of the glaring face of its parent-star. Another indirect method--the radial velocity method--depends on the detection of a tiny wobble that an orbiting planet induces on its star. Both the transit method and the radial velocity method favor the discovery of massive planets that are situated close to their searing-hot, fiery parent-star--rather than smaller Earth-like worlds that circle their star at a greater--and more comfortable--distance.