KNOW KNOW

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STAR PL ANET

Exoplanets come in a wide variety of sizes, from gas giants larger than Jupiter to small, rocky planets about as big around as Earth or Mars. They can be hot enough to boil metal or locked in deep freeze. They can orbit their stars so tightly that a “year” lasts only a few datys; they can orbit two suwwwns at once. Some exoplanets are sunless rogues, wandering through the galaxy in permanent darkness. Credits: NASA/W. Stenzel Everything we derive Not only do apples fall close to the so far — and even they require, first, tree, but the tree’s history can strongly the detection of the star itself. Can the with regard to the influence the taste of the apple. “We’re now entering this era of really star fool us characteristics of the still getting its bearings, just a trying to understand the structure into planet — the size of few months after launch, when the and composition of the planets, trying exhibiting variability that of exoplanet discovery, insuffi- floodgates burst open. In reality, we to understand what kinds of systems thinking can masquerade as a planet cient understanding of stellar the planet, the mass, can’t even find most planets outside our planets can exist in,” said astronomer ʺ Many stars move in signal,” said Jennifer Burt, rotation led to “false positives” the atmosphere — is solar system — exoplanets — without David Ciardi of NASA’s Exoplanet it has a orbital duets with companion a postdoctoral exoplanet – signals that at first appeared help from their stars. Every planet Science Institute (NExScI) at Caltech. stars, which can look a lot all done relative to planet researcher at NASA’s Jet to be planets, but actually detection method known requires “The star is the most dominant part of like planets. Others play Propulsion Laboratory in came from other sources the star,” Ciardi says. a detailed dossier on the star, with a solar system; it has the most mass, that host to giant objects called Pasadena, California. upon closer inspection. Those “You need to know very few exceptions: finding “rogue the most energy influence. We’re brown dwarfs, a kind of Among these variations: exoplanet announcements planets” that mysteriously wander studying these systems holistically isn’t really “failed star” that is considered star spots (our Sun’s version were then withdrawn. the star in order to the galaxy without stellar compan- — not just an individual rock.” neither a star nor a planet. there? are called sunspots). “The It’s a major problem for know the planet. ions, and planets that are directly And failure to properly star’s rotation period dictates one of the most intriguing imaged – capturing pixels of light account for a star’s pulses, how star spots rotate on and classes of exoplanets: rocky, from the planets themselves. These jitters and other variations off the side of the star we can M-dwarfs. ture for liquid directly imaged planets represent can lead to a vexing prob- see from Earth,” Burt said. water to pool on the surface. only a tiny fraction of the thousands of lem: phantom planets. Especially in the early days exoplanets discovered in our galaxy “The star itself is often also 2 SPACE TIME March 2021 3 Artist's illustration of NASA's Transiting Exoplanet Survey Satellite (TESS). (Image credit: NASA's Goddard Space Flight Center)

Or we can view the other side of the question: Without needed measurements, can the star fool us

into thinking it has a planet that isn’t really there?

Many stars move in orbital duets with

companion stars, which can look a lot like

planets. Others play host to giant objects called

brown dwarfs, a kind of “failed star” that is

considered neither a star nor a planet. ? And failure to properly account for a star’s

pulses, jitters and other variations can lead

to a vexing problem: phantom planets.

“The star itself is often also exhibiting

variability that can masquerade as a planet “It’s interesting to have stellar 10 days, and “10 days is not elemental abundances to go along unusual for the rotation period signal,” said Jennifer Burt, a postdoctoral ant to know an exoplanet’s W with those of the planet, as the of an M-dwarf,” he said. If a star's exoplanet researcher at NASA’s Jet Propul- temperature? Unless it is so hot combination informs theories of rotation takes about the same time that it’s emitting its own light, planet formation and evolution,” as a planet's orbit, it's difficult to sion Laboratory in Pasadena, California. you’ll need to know its orbital dis- Stapelfeldt said. “In both cases, tell the two apart. It's still possible Among these variations: star spots (our Sun’s tance from the star (see the transit the abundances are derived from to confirm the planet is there; version are called sunspots). “The star’s rotation method) and the star’s luminosity, taking a spectrum” — that is, it just takes a lot more work. or how much sheer power the star analyzing the spectrum of light Bottom line: The star is in period dictates how star spots rotate on and off the radiates (think light-bulb wattage). from stars and planets to reveal charge. It's why even careful study side of the star we can see from Earth,” Burt said. Learning how big around elements in their atmospheres. of our own sttar, the Sun, can help the planet is, what it’s made However, the rotation period us better understand exoplanets. Especially in the early days of exoplanet of, even the composition of its of the star can be similar to the WHAT IS AN EXOPLANET EXOPLANET AN IS WHAT discovery, insufficient understanding of stellar atmosphere — you guessed it. orbital period of planets in the rotation led to “false positives” – signals that at Knowledge of the star is critical. habitable zones, according to Planets generally form from Eric Mamajek, deputy program first appeared to be planets, but actually came the same cloud of gas and dust scientist for NASA’s Exoplanet from other sources upon closer inspection. Those as their host star. So elements Exploration Program. found in a star’s atmosphere have A year on such planets — once exoplanet announcements were then withdrawn. direct bearing on its planets. around the star — might take It’s a major problem for one of the most intriguing

classes of exoplanets: rocky, Earth-sized worlds

that orbit within the “habitable zone” of red-dwarf

stars, also called M-dwarfs. If such planets possess

atmospheres, some could be at just the right

temperature for liquid water to pool on the surface.

The seven planets of TRAPPIST-1 form a system

with multiple Earth-size worlds in this special zone.

However, the rotation period of the star can be sim-

ilar to the orbital period of planets in the habitable

zones, according to Eric Mamajek, deputy program

scientist for NASA’s Exoplanet Exploration Program.

This animation shows a type of gas giant planet known as hot Jupiter that orbits very close to its star. NASAJPL-Caltech

4 SPACE TIME March 2021 5 f you want to get to know a faraway planet Ibetter, say a small, rocky world tens or hundreds of light-years away, you’d best ʺstart by getting to know its star. By Pat Brennan

In a galaxy 100 light-years away in the constellation Dorado sits a planetary system named TOI 700. It is home to TOI 700 d, the first Earth-size habitable-zone planet discovered by TESS, NASA’s Transiting Exoplanet Survey Satellite.

Image Credit: NASA/Goddard Space Flight Center

This illustration depicts Kepler-186f, the first validated Earth-size planet to orbit a distant star in the habitable zone.

Image Credit: NASA Ames/JPL-Caltech/T. Pyle

6 SPACE TIME March 2021 7