The more we learn about the billions upon billions of planets that orbit beyond our solar system, the more we are surprised by the wild menagerie of objects out there. From the start, many of these untolled planets have been startling, paradigm-breaking, mysterious, hellish, potentially habitable and just plain weird. Despite the confirmed detection of more than 4,000 exoplanets, the job of finding and characterizing these worlds remains in its early phases. You could make the argument that learning a lot more about these distant exoplanets and their solar systems is not just one of the great tasks of future astronomy, but of future science.
And that is why Many Worlds is returning to the subject of “Weird Planets,” which first appeared in this column at the opening of 2019. It has been the most viewed column in our archive, and a day seldom goes by without someone — or some many people — decide to read it.
So here is not a really a sequel, but rather a continuation of writing about this unendingly rich subject. And as I will describe further on, almost all of the planets on display so far have been detected and characterized without ever having been seen. The characteristics and colors presented in these (mostly) artistic renderings are the result of indirect observing and discovery — measuring how much light dims when a faraway planet crosses its host star, or how much the planet’s gravity causes its sun to move.
As a result, these planets are sometimes called “small, black shadows.” Scientists can infer a lot from the indirect measurements they make and from the beginnings of the grand effort to spectroscopically read the chemical makeup of exoplanet atmospheres. But different technologies and approaches are on the way, or at least are being eagerly planned.
The newest space telescope contributing to exoplanet knowledge is NASA’s Transiting Exoplanet Survey Satellite or TESS. It is not an especially elaborate (or costly) observatory, but it has a compelling mission: It will perform an all sky survey in search of bright, nearby stars. In this way, it will be quite different from the iconic Kepler Space Telescope which performed a census of exoplanets distant from us and forever changed our notion of how many exoplanets might be out there.
Launched in 2018, intriguing results from TESS are coming in now. For instance, TESS Object of Interest (TOI) 561 belongs to a rare population of stars called the galactic thick disk. Stars in this region are chemically distinct, with fewer heavy elements such as iron or magnesium that are created in massive supernova explosions and are associated with planet building. To astronomers, the paucity of these heavy metals means the stars are very old and any planets detected around them would be very old, too.
“TOI-561b is one of the oldest rocky planets yet discovered,” said University of Hawaii postdoctoral fellow and team lead Lauren Weiss, who estimated it was formed about 10 billion years ago. “Its existence shows that the universe has been forming rocky planets almost since its inception 14 billion years ago.”
The planet has an short orbit because of its proximity to its star, which creates incredible heat. Its estimated average surface temperature is over 2,000 degrees Kelvin — much too toasty to host life as we know it today, though it may once have been possible.
As explained by University California, Riverside astrophysicist Stephen Kane, although the planet has roughly three times the mass of Earth, the team calculated its density to be the same as our planet.
“This is surprising because you’d expect the density to be higher,” Kane said. “This is consistent with the notion that the planet is extremely old.”
The TESS Mission team used the University of California’s access to the W.M. Keck Observatory in Hawaii — home to some of the most scientifically productive telescopes on Earth — to confirm the presence of planet TOI-561b. Because the TESS observatory is highly limited in its onboard instruments, Keck equipment was used to helped the team calculate the planet’s mass, density and radius.
TESS finds planets and characterizes them as it can via the transit method, where small dips in light from the star are measured and translated into the presence and qualities of a planet passing in front of it.
This type of exoplanet discovery will continue for rather a while, but on the horizon is a move to direct imaging, where telescopes actually image the planet (and host star) rather than measure the indirect signatures of the presence of planets. This requires telescopes with much higher resolution and with coronagraphs to block out the blinding sun of host stars.
A small number of exoplanets have been directly imaged from ground telescopes so far but, because of limits on the technology available and the need to compensate for the effects of our own atmosphere, they are all very large gas planets (Jupiters and beyond) and far from the host stars. No potentially habitable planets, no small rocky planets. But that is expected to change in the years ahead.
The Roman Space Telescope (formerly known as WFIRST) is being developed to have direct imaging capacities and the first demonstration coronagraph potentially capable of observing relatively smaller Neptune-sized planets (though not Earth-sized planets.) The project has been around for quite a while — it was recommended by the National Academies of Science Decadal Survey as a Flagship mission in 2010 — but the administration has sought to cancel it and it has been kept alive but not thriving by Congress.
In the meantime, ground-based direct imaging demonstrations have been going on at sites such as the Gemini Planet Imager (GPI), a high contrast imaging instrument that was built for the Gemini South Telescope in Chile. GPI has identified a handful of large planets directly.
The era of in-space direct imaging, however, is on its way. The James Webb Space Telescope, scheduled to launch later this year, does have some direct imaging capacity and it can be used for some exoplanet work. The observatory, however, sees in the infrared wavelength and so has limits for exoplanet research
What many exoplanet scientists are really pushing for is a grand observatory for the 2030s designed first with exoplanet research and then other astrophysics in mind. Two proposals are now among the four before the Decadal Survey for Astronomy and Astrophysics (ASTRO 2020) would revolutionize space-based direct imaging as well as exoplanet hunting and exploring. The two proposals are called LUVOIR (Large UV/Optical/IR Surveyor) and HabEx (Habitable Planet Observatory), and the Decadal recommendation is expected in the spring.
What makes LUVOIR in particular so extraordinary is that it is proposed to have an enormous telescope mirror — as wide as 50 feet in diameter. HabeX is proposed at 13-feet. Both would have very high power coronagraphs to block out the host sun in the systems they are observing and so they could see smaller planets and those much closer to their suns. In contrast, the grand workhorse Hubble Space Telescope has a 7.5-foot mirror diameter and no coronograph.
Back in our own solar system, here is a comparison of what Hubble sees when it looks for the intriguing plumes coming off Jupiter’s moon Europa and what the proposed LUVOIR would see. Clearly, a new world of weird and wonderful planets would become available for us (and scientists, of course) to appreciate. Direct imaging especially of the LUVOIR scale, and to a lesser extent HabEx, too, is costly for sure and may slow down its arrival. But the advantages to astronomy and exoplanet research is too good to ignore forever.
Meanwhile, the way forward often involves a look back. The much-discussed planet Kepler- 452b , for instance, may not be a planet at all.
In 2015, NASA declared that Kepler-452b was the first near-Earth-sized planet orbiting in the “habitable” zone around a star very similar to our sun. The space agency called it Earth’s “bigger, older cousin,” and scientists were so enthusiastic that one began quoting poetry at a news conference.
But new information analyzed by an astronomer formerly with the Kepler science team raised serious questions about whether the “finding” was actually a planet. To become a confirmed exoplanet, generally scientists like to see a confidence level of about 99 percent. But for K-452b, says astronomer Fergal Mullally, the confidence level is somewhere between 50 and 90 percent.
But Mullally says don’t be too disappointed if K-452 is not actually a planet because there are so many others out there. One of his favorites: One of his favorites is Kepler-442b, a world that’s likely rocky and with temperatures suitable for life. It orbits a star a little cooler than our sun, so it’s not exactly like Earth. Co-author Christopher Burke says its discovery remains solid — and is reliably confirmed.
Shortly after NASA’s Kepler mission began operations back in 2009, the space telescope spotted what was thought to be a planet about half the size of Saturn in a multiple-star system. KOI-5Ab was only the second planet candidate to be found by the mission, and exciting as it was at the time, it was ultimately set aside as Kepler racked up more and more planet discoveries.
By the end of the spacecraft’s operations in 2018, Kepler had discovered a whopping 2,394 exoplanets, or planets orbiting stars beyond our sun, and an additional 2,366 exoplanet candidates that would still need confirmation.
“KOI-5Ab got abandoned because it was complicated, and we had thousands of candidates,” said David Ciardi, chief scientist of NASA’s Exoplanet Science Institute. “There were easier pickings than KOI-5Ab, and we were learning something new from Kepler every day, so that KOI-5 was mostly forgotten.”
Now, after a lengthy hunt that spanned many years and many telescopes, Ciardi said he has “resurrected KOI-5Ab from the dead.” Thanks to new observations from NASA’s second planet-hunting mission, the Transiting Exoplanet Survey Satellite, or TESS, and a number of ground-based telescopes, Ciardi was finally able to untangle all the evidence surrounding KOI-5Ab and prove its existence. There are some intriguing details about it to mull over.
Most likely a gas giant planet like Jupiter or Saturn in our solar system given its size, KOI-5Ab is unusual in that it orbits a star in a system with two other companion stars, circling on a plane that’s out of alignment with at least one of the stars. The arrangement calls into question how each member in this system formed out of the same swirling clouds of gas and dust. Ciardi, who is located at Caltech in Pasadena, California presented the findings at a virtual meeting of the American Astronomical Society.
“We don’t know of many planets that exist in triple-star systems, and this one is extra special because its orbit is skewed,” said Ciardi. “We still have a lot of questions about how and when planets can form in multiple-star systems and how their properties compare to planets in single-star systems. By studying this system in greater detail, perhaps we can gain insight into how the universe makes planets.”
For a finale, I would like to return to the Transiting Planets and Planetesimals Small Telescope (Trappist-1 ) solar system I wrote about in the original “Weird Planets” column.
This is one of the more remarkable and, of late, more studied of solar systems because it has seven rocky, Earth-sized planets orbiting it. with three of them apparently in the system’s “habitable zone.” The planets orbit a Red Dwarf star, a type that is much “cooler” than Sun-like stars and is the most common type of star in the cosmos. The Trappist-1 system is pretty close to us at 40 light years away.
When I first wrote about it, I just made a passing reference to the fact that the seven planets appear to be in resonance with one another. This means that that the orbital periods of each can be expressed as ratios of two integers. For example, two planets orbiting a parent star are said to be in 2:1 resonance when one of the planets takes approximately twice as long to orbit the sun as the other planet.
A number of the moons of both Jupiter and Saturn are known to be in resonance with each other, and a number of planetary systems around other suns are also suspected to harbor planets in resonance.
But the seven resonant planets of the TRAPPIST-1 planets are very unusual (as far as we know) in terms of having so many objects orbiting in near resonance — each one resonant with the others. For example, for every two orbits of TRAPPIST-1h (the outermost planet in the system), the other six worlds orbit the host star three, four, six, nine, 15 and 24 times, respectively.
The Trappist-1 system is thought to be 3 to 8 billion years old, and the continued existence of the seven planets had been something of a mystery. Computer simulations found that the planets should have started slamming into each other less than 1 million years after their formation.
But resonance — established and maintained by gravitational pulls — apparently saved them. The planets were likely able to settle into their system-stabilizing resonant orbits soon after they formed, according to a 2017 in Astrophysical Journal Letters led by Dan Tamayo, a postdoctoral researcher at the University of Toronto’s Centre for Planetary Sciences. This was a follow-up and expansion of the work done by the initial founders of the Trappist-1 planets, a team led by Michaël Gillonhe University of Liège in Belgium.