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.
As of January 1, 2020, there are 4,160 confirmed exoplanets inhabiting 3,090 systems, with 676 systems hosting more than one solitary planet.
Astronomers confirmed TESS’s discovery of TOI-700 d using NASA’s infrared Spitzer Space Telescope, and they have created computer models of the planet’s potential environments to help inform future missions.
TOI-700 d has the important distinction of being one of only a few Earth-size planets discovered so far to be in orbit within its parent-star’s habitable Goldilocks zone. Others include several planets dwelling within the TRAPPIST-1 system, as well as some other distant worlds discovered by NASA’s Kepler Space Telescope.
“TESS was designed and launched specifically 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. Discovering TOI-700 d is a key science finding for TESS. Confirming the planet’s size and habitable zone status with Spitzer is another win for Spitzer as it approaches the end of science operations this January,” explained Dr. Paul Hertz in a January 6, 2020 NASA Jet Propulsion Laboratory (JPL) Press Release. Dr. Hertz is astrophysics director at NASA Headquarters in Washington. The JPL is in Pasadena, California.
TESS monitors large swaths of the sky, which are termed sections, for 27 days at a time. This continuous, long stare enables the satellite to detect alterations in stellar brightness that are caused by an orbiting planet, floating in front of the glaring face of its star, from our perspective (transit). Astronomers detected multiple transits by TOI-700’s trio of planets.
TOI-700 was originally misclassified in the TESS database as being a star more similar to our own Sun, rather than the smaller, cooler red dwarf star that it turned out to be. This means that at first the orbiting trio of planets appeared to be larger and hotter than they actually are. Several researchers, including Alton Spencer, a high school student working with members of the TESS team, discovered the mistake.
“When we corrected the star’s parameters, the sizes of the planets dropped, and we realized the outermost one was about the size of Earth and in the habitable zone. Additionally, in 11 months of data we saw no flares from the star, which improves the chances TOI-700 d is habitable and makes it easier to model its atmospheric and surface conditions,” Emily Gilbert noted in the January 6, 2020 JPL Press Release. Gilbert is a graduate student at the University of Chicago.
Ms. Gilbert and other scientists presented the findings at the 235th meeting of the American Astronomical Society (AAS) held in Honolulu, Hawaii in January 2020. Three papers describing the new findings–one of which was led by Ms. Gilbert–have been submitted to scientific journals.
The innermost of the trio of planets, dubbed TOI-700 b, is almost exactly the same size as Earth. It is probably a rocky world that completes an orbit every 10 days. 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 likely a gaseous world. TOI-700 d, the outermost known planet inhabiting the system and the only one situated in the Goldilocks habitable zone, measures 20% larger than Earth, and it orbits its star every 37 days. TOI-700 d receives 86% of the energy from its stellar parent that the Sun provides to Earth. All three planets are thought to be tidally locked to their star. This means they rotate once per orbit so that one side constantly basks in the brightness of daylight.
A team of astronomers led by Dr. Joseph Rodriguez, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA), in Cambridge, Massachusetts, requested follow-up observations with Spitzer to confirm TOI-700 d.
“Given the impact of this discovery–that it is TESS’s first habitable zone Earth-size 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. 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 commented in the January 6, 2020 JPL Press Release.
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% and its size by 36%. In addition, it ruled out other possible astrophysical sources of the transit signal, such as the existence of a smaller, fainter companion star lurking in the system.
Dr. Rodriguez and his team also used follow-up observations obtained from a 1-meter ground-based telescope in the global Las Cumbres Observatory network to improve astronomers’ confidence in the orbital period and size of TOI-700 c by 30% and 36%, respectively.
Because TOI-700 displays no sign of stellar flares, is bright, and close by, the system is a prime target for exact mass measurements by ground-based observatories that are currently available. These measurements could potentially validate astronomers’ estimates that the inner and outer planets circling this small red dwarf are rocky and that the middle planet is composed of gas.
Future missions may be capable of determining whether the trio of planets have atmospheres–and, if they do, even be able to identify their compositions.
Even though the exact conditions on TOI-700 d are currently unknown, astronomers can use the information that is currently available to create models and make predictions. The information now available indicates both the size and the type of star it orbits. Astronomers at NASA’s Goddard Space Flight Center (GSFC) in Greenbelt, Maryland, created models of 20 potential environments of TOI-700 d to determine if any version would result in surface temperatures and pressures that would make it habitable.
Their 3D climate models examined a variety of atmospheric compositions and surface types normally associated with what scientists consider to be potentially habitable worlds. Because TOI-700 d is tidally locked to its stellar parent, the planet’s wind patterns and cloud formations may be very different from those on our own planet.
One simulation included an ocean-covered TOI-700 d. That model also included a dense, carbon-dioxide-dominated atmosphere for this distant world. This type of atmosphere is similar to the one that many scientists propose surrounded Mars when it was young. The model atmosphere also sports a deep layer of clouds on the star-facing side. Another model portrays TOI-700 d as a cloudless world, that is an all-land version of Earth. On this type of world, winds rush away from the night side of the planet, and then converge on the point directly facing the glare of the parent-star.
When starlight flows through a planet’s atmosphere, it dances with molecules like carbon dioxide and nitrogen to form distinct signals. These signals are termed spectral lines. The team of modeling scientists, led by Dr. Gabrielle Englemann-Suissa, a Universities Space Research Association visiting research assistant at Goddard, produced simulated spectra for the 20 modeled versions of TOI-700 d.
“Someday, when we have real spectra from TOI-700 d, 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 completely diffeent from what we have here on Earth,” Dr. Englemann-Suissa told the press on January 6, 2020.