Artist's rendering of a Jupiter-sized planet and its host star.

Know the Star, Know the Planet

Artist's rendering of a Jupiter-sized planet and its host star. Image credit: ESO.

Feature | May 12, 2020

Not only do apples fall close to the tree, but the tree’s history can strongly influence the taste of the apple.

Something similar can be said for planets. If you want to get to know a faraway planet better, say a small, rocky world tens or hundreds of light-years away, you’d best start by getting to know its star.

In reality, we can’t even find most planets outside our solar system — exoplanets — without help from their stars. Every planet detection method known requires a detailed dossier on the star, with very few exceptions: finding “rogue planets” that mysteriously wander the galaxy without stellar companions, and planets that are directly imaged – capturing pixels of light from the planets themselves. These directly imaged planets represent only a tiny fraction of the thousands of exoplanets discovered in our galaxy so far — and even they require, first, the detection of the star itself.

“We’re now entering this era of really trying to understand the structure and composition of the planets, trying to understand what kinds of systems planets can exist in,” said astronomer David Ciardi of NASA’s Exoplanet Science Institute (NExScI) at Caltech. “The star is the most dominant part of a solar system; it has the most mass, the most energy influence. We’re studying these systems holistically — not just an individual rock.”

Graphic of exoplanet types

If you put the question to Karl Stapelfeldt, chief scientist for NASA’s Exoplanet Exploration Program, he’ll simply run down the list of exoplanet detection methods that require intimate knowledge of the host star:

  • Radial velocity — measuring the wobbles in the movement of a star caused by gravitational tugs from an orbiting planet — can reveal the mass, or heft, of the target exoplanet. But that only works if you know, to high accuracy, the mass of the star.
  • The transit method — looking for a tiny dip in starlight as a planet crosses the face of its star — can tell you the length of the planet’s year, or once around the star, by watching how often the dip repeats. But knowing the size of that orbit, or the planet’s distance from the star, requires measurement of a star’s mass. The star’s diameter is needed as well; then the size of the dip in starlight will reveal the size of the planet.
  • It’s the same story for other detection methods, including gravitational microlensing — using magnification of light from a background star to reveal a star and its exoplanet in the foreground — and astrometry, another way of finding exoplanets by tracking stellar motions.

Seeing by starlight

Such star measurements are indispensable for NASA’s newest space-based planet hunter, TESS (the Transiting Exoplanet Survey Satellite). They can reveal even more about planets when combined with asteroseismology, or the measurement of “star quakes.”

“The [starlight measurements] we get from TESS can help provide precise stellar radii for the brightest stars via asteroseismology,” said Knicole Colón, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and a member of the TESS team. “Knowing the sizes of stars lets us measure sizes of planets precisely. TESS data can also help us measure ages of stars, in turn providing an estimate of the ages of planets.”

Want to know an exoplanet’s temperature? Unless it is so hot that it’s emitting its own light, you’ll need to know its orbital distance from the star (see the transit method) and the star’s luminosity, or how much sheer power the star radiates (think light-bulb wattage).

Learning how big around the planet is, what it’s made of, even the composition of its atmosphere — you guessed it. Knowledge of the star is critical. Planets generally form from the same cloud of gas and dust as their host star. So elements found in a star’s atmosphere have direct bearing on its planets.

“It’s interesting to have stellar elemental abundances to go along with those of the planet, as the combination informs theories of planet formation and evolution,” Stapelfeldt said. “In both cases, the abundances are derived from taking a spectrum” — that is, analyzing the spectrum of light from stars and planets to reveal elements in their atmospheres.

Planetary imposters?

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 signal,” said Jennifer Burt, a postdoctoral exoplanet researcher at NASA’s Jet Propulsion Laboratory in Pasadena, California.

Among these variations: star spots (our Sun’s version are called sunspots). “The star’s rotation period dictates how star spots rotate on and off the side of the star we can see from Earth,” Burt said.

Especially in the early days of exoplanet discovery, insufficient understanding of stellar rotation led to “false positives” – signals that at first appeared to be planets, but actually came from other sources upon closer inspection. Those exoplanet announcements were then withdrawn.

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.

Seven planets of the TRAPPIST-1 system
Artist's rendering of the TRAPPIST-1 planetary system. Image credit: NASA/JPL-Caltech

However, the rotation period of the star can be similar to the orbital period of planets in the habitable zones, according to Eric Mamajek, deputy program scientist for NASA’s Exoplanet Exploration Program.

A year on such planets — once around the star — might take 10 days, and "10 days is not unusual for the rotation period of an M-dwarf," he said.

If a star's rotation takes about the same time as a planet's orbit, it's difficult to tell the two apart. It's still possible to confirm the planet is there; it just takes a lot more work.

Bottom line: The star is in charge. It's why even careful study of our own sttar, the Sun, can help us better understand exoplanets.

"Everything we derive with regard to the characteristics of the planet — the size of the planet, the mass, the atmosphere — is all done relative to the star," Ciardi says. "You need to know the star in order to know the planet."