Astronomers have learned to predict the unpredictable since the first exoplanet was found a century ago. A wonderful treasure chest of weird Fantasy Worlds has been uncovered for over twenty years. Yes, some of these very strange worlds are so odd in orbit around stars outside our Sun, that scientists never believed anything like them might possibly occur in the Cosmos — that is, before they were found. Aside from unusual alien planets, the Holy Grail of planet-hunting astronomy has long been to discover more homelike organisms.
Astronomers reported in January 2020 the detection of just such a long-sought-for world — the first to be detected by the Transiting Exoplanet Survey Satellite (TESS) of NASA. The remote planet Earth-size is safely located in the habitable zone of the star The habitable zone of a star is the “Goldilocks” spectrum of wavelengths where temperatures are not too dry, not too cold, so “about perfect” to pool liquid water on the floor. If liquid water does occur, there might still be existence as we know it.
The Earth-like planet, called TOI-700 d, circles a tiny red dwarf star dubbed TOI-700, which in the Dorado constellation is just 101.4 light-years distant. That star is the brightest known stellar host of a transiting habitable zone, Earth-size world. The acronym “TOI” refers to the TESS-studied stars and exoplanets. The red dwarf star, TOI-700, is of spectral class M, with a mass of 40%, a radius of 40%, and a temperature of 50% of our Sun.
The light star displays weak stellar intensity rates too. Red dwarf stars in our Milky Way Galaxies are the biggest — as well as the most common — genuine nuclear-fusing planets. They will “last” for trillions of years, as they are so tiny and sweet. Our slightly larger Sun, by contrast, can only “live” for 10 billion years. Rather large stars will only “last” for millions of years because their immense heat forces them to exhaust their nuclear fuel supplies quicker than their smaller stellar relatives. The bigger the star, its “life” shorter.
An exoplanet was first formally observed in 1988. After that, in 1992, the first confirmed observation was made with the identification of many terrestrial-mass planets in orbit around the PSR B1257 + 12 pulsars. A pulsar is the remains of a massive star that ended its ‘life’ in a supernova blast of a core-collapse (Type II). Pulsars are young neutron stars that are born spinning rapidly with a regularity frequently likened to a lighthouse beacon on Earth. They are items of city-scale which are so large that a teaspoon full of their substance will weigh as much as a thundering herd of wild horses. Such infant neutron stars are just one massive atomic nucleus. A pulsar was one of the last planetary bodies that scientists felt would play host to a family of planets — that is, before they were found. The pulsar planets were the first of a long string of oddball exoplanet findings. They are hostile little worlds, mercilessly showered with the deadly radiation beams of their parent-pulsar.
An exoplanet, circling a “natural” hydrogen-burning star like our Sun, was first observed in 1995. Also, this new discovery proved to be a surprising odd — a giant planet surrounding its searing-hot stellar parent quickly and close. The planet, Pegasi 51 b, is in a 4-day roasting orbit around its star, Pegasi 51. As it turned out this large planetary “roaster” was the first to be found in a fresh and unexpected exoplanet class — hot Jupiters. After the observation of 51 Peg b, dozens of others of its peculiar form have been found in orbit around stars outside our Sun.
Many of the exoplanets were specifically imaged by telescopes. However, the overwhelming majority have been observed by indirect means, such as the transit process, through which a planet is detected spinning in front of its parent star’s blinding nose. Another indirect method — the method of radial velocity — depends on the detection of a small wobble induced by an orbiting planet upon its star. Both the method of transit and the method of radial velocity favors the discovery of massive planets near their searing-hot, fiery parent-star — rather than smaller Earth-like worlds which circle their star at a greater — and more comfortable — distance.
As of 1 January 2020, there are 4,160 confirmed exoplanets inhabiting 3,090 systems, with more than one solitary planet hosting 676 systems.
Astronomers verified the detection of TOI-700 d by TESS utilizing NASA’s infrared Spitzer Space Telescope, producing computational simulations of the possible conditions of the earth to better guide prospective missions.
TOI-700 d has the important distinction of being one of only a few planets of Earth-size so far discovered to be in orbit within the habitable Goldilocks zone of its parent star. Others include several planets that live within the TRAPPIST-1 system, as well as some other distant worlds that NASA’s Kepler Space Telescope has discovered.
“TESS was specifically designed and launched to find Earth-sized planets orbiting nearby stars. Planets around nearby stars are easiest to follow up with target telescopes in space and on Earth. The discovery of TOI-700 d is a key science finding for TESS. Confirming the size and living area status of the planet with Spitzer is another victory for Spitzer as it approaches the end of scientific operations. Dr. Hertz is chief of astrophysics at NASA headquarters in Washington, DC. JPL is located in Pasadena, California.
For 27 days at a time, TESS monitors large swaths of the sky, which are called sections. This constant, long gaze helps the spacecraft to track shifts in stellar brightness induced by an orbiting object, floating from our view (transit) until the star’s flashing nose. Astronomers detected multiple transits through the trio of planets in TOI-700.
In the TESS database, the TOI-700 was initially misclassified as a star much like our own Planet, rather than the tiny, cooler red dwarf star it turned out to be. This indicates that the spinning group of planets appear at first to be smaller and cooler than they really are. The mistake was discovered by several researchers, including Alton Spencer, a high school student who works with TESS team members.
“As we changed the parameters of the star, the sizes of the planets fell, and we found that the outermost one was around the size of the Earth and in the habitable zone. Moreover, in 11 months of observations we observed no starbursts, which increases the likelihood that TOI-700 d is stable and allows it possible to model the atmosphere and surface conditions,” reported Emily Gilbert on January 6, 2020, JPL P. Gilbert is University of Chicago graduate student.
During the 235th conference of the American Astronomical Society (AAS), conducted in Honolulu, Hawaii in January 2020, Mrs. Gilbert and other scientists discussed the results. Three papers have been submitted to scientific journals describing the new findings-one of which was led by Ms. Gilbert.
The innermost planetary trio, dubbed TOI-700 b, is almost exactly the same size as Earth. It is probably a rocky world that completes a 10-day orbit. The middle planet, named TOI-700 c, is 2.6 times larger than Earth — between the sizes of Earth and Neptune. TOI-700 c orbits its parent-star every 16 days and is possibly a gaseous planet. TOI-700 d, the outermost known planet inhabiting the system and the only one in the habitable area of the Goldilocks, is 20 percent larger than Earth and orbits its star every 37 days. TOI-700 d derives 86 percent of the energy that the Sun gives to Earth from its celestial relative. It’s thought that all three planets are tidally locked to their star. This means that they rotate once per orbit so that one side is constantly basking in daylight brightness.
A team of astronomers led by Dr. Joseph Rodriguez, an astronomer at the Harvard-Smithsonian Astrophysics Center (CfA) in Cambridge, Massachusetts, asked Spitzer for follow-up observations to confirm TOI-700 d.
“Because of the impact of this discovery — that it is the first living area of TESS Earth-sized planet — we really wanted our understanding of this system to be as concrete as possible. Spitzer saw TOI-700 d transit exactly when we expected it to be. It’s a great addition to the legacy of a mission that helped confirm two of the TRAPPIST-1 planets and identify five more,” Dr. Rodriguez said in January.
The Spitzer data increased astronomers’ confidence that TOI 700 d is truly a planet, and also rendered more precise their measurements of its orbital period by 56 percent and its size by 36 percent. Additionally, it ruled out other possible transit signal astrophysical sources, such as the existence of a smaller, fainter companion star lurking within the system.
Dr. Rodriguez and his team also used follow-up observations from a 1-meter ground-based telescope in the global Las Cumbres Observatory network to improve astronomer confidence by 30 percent and 36 percent respectively during the orbital period and the size of TOI-700 c.
Because TOI-700 displays no signs of stellar flares, is bright, and close by, the system is a prime target for precise mass measurements by currently available ground-based observatories. These measurements could potentially validate the estimates of astronomers that the inner and outer planets surrounding this little red dwarf are rocky, and that the middle planet is made up of gas.
Future missions will be able to assess if the planet’s trio has atmospheres — and also be able to classify their compositions if they are.
While the exact conditions are still uncertain on TOI-700 d, astronomers may use the knowledge still available to build models and make predictions. The accessible knowledge now shows both the scale and shape of the star it circles. Astronomers at NASA’s Goddard Space Flight Center (GSFC) in Greenbelt, Maryland, have created models of 20 potential TOI-700 d environments to determine whether any version would result in surface temperatures and pressures making it habitable.
Their 3D climate models examined a variety of atmospheric compositions and surface types which are normally associated with what scientists see as potentially habitable worlds. Because TOI-700 d is tidally locked to its stellar parent, the wind patterns of the planet and cloud formations may differ greatly from those on our planet itself.
One test featured a TOI-700 d, surrounded by the ocean. In this distant planet, too, the paradigm featured a thick, carbon-dioxide-dominated environment. This form of the atmosphere is identical to the one suggested by several scientists when it was young, covering Mars. The layout atmosphere often sports the star-facing hand to a dense layer of clouds. Another model portrays TOI-700 d as a cloudless world which is an all-land Earth version. In this kind of universe, winds surge out from the planet’s night side, then align to the point immediately facing the parent star’s glare.
As starlight passes across the atmosphere of a world it moves to create distinct messages using molecules such as carbon dioxide and nitrogen. Those are also spectral lines. The team of modeling scientists led by Dr. Gabrielle Englemann-Suissa, a Universities Space Research Association visiting Goddard’s research assistant, produced simulated spectra for the 20 TOI-700 d model versions.
“Once we have real TOI-700 d spectra, we can backtrack and then match that to a model. It’s exciting because no matter what we find out about the planet, it’s going to look quite different from what we have here on Earth,” Dr. Englemann-Suissa told the press on January 6, 2020.