At Last, Scientists Have Found The Galaxy’s Missing Exoplanets: Cold Gas Giants

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There are four known exoplanets orbiting the star HR 8799, all of which are more massive than the planet Jupiter. These planets were all detected by direct imaging taken over a period of seven years, with the periods of these worlds ranging from decades to centuries.

Jason Wang / Christian Marois

In the early 1990s, scientists began detecting the first planets orbiting stars other than the Sun: exoplanets. The easiest ones to see had the largest masses and the shortest orbits, as those are the planets with the greatest observable effects on their parent stars. The second types of planets were at the other extreme, massive enough to emit their own infrared light but so distant from their star that they could be independently resolved by a powerful enough telescope.

Today, there are over 4,000 known exoplanets, but the overwhelming majority either orbit very close to or very far from their parent star. At long last, however, a team of scientists has discovered a bevy of those missing worlds: at the same distance our own Solar System’s gas giants orbit. Here’s how they did it.

In our own Solar System, the planets Jupiter and Saturn produce the greatest gravitational influence on the Sun, which will lead to our parent star moving relative to the Solar System’s center-of-mass by a substantial amount over the timescales it takes those giant planets to orbit. This motion results in a periodic redshift and blueshift that should be detectable over long enough observational timescales.

NASA’s The Space Place

When you look at a star, you’re not simply seeing the light it emits from one constant, point-like surface. Instead, there’s a lot of physics going on inside that contributes to what you see.

  • the star itself isn’t a solid surface, but emits the light you see for many layers going down hundreds or even thousands of kilometers,
  • the star itself rotates, meaning one side moves towards you and the other away from you,
  • the star has planets that move around it, occasionally blocking a portion of its light,
  • the orbiting planets also gravitationally tug on the star, causing it to periodically “wobble” in time with the planet orbiting it,
  • and the star moves throughout the galaxy, changing its motion relative to us.

All of these, in some way, matter for detecting planets around a star.

At the photosphere, we can observe the properties, elements, and spectral features present at the outermost layers of the Sun. The top of the photosphere is about 4400 K, while the bottom, 500 km down, is more like 6000 K. The solar spectrum is a sum of all of these blackbodies, and every star we know of has similar properties to their photospheres.

NASA’s Solar Dynamics Observatory / GSFC

That first point, which might seem the least important, is actually vital to the way we detect and confirm exoplanets. Our Sun, like all stars, is hotter towards the core and cooler towards the limb. At the hottest temperatures, all the atoms inside the star are fully ionized, but as you move to the outer, cooler portions, electrons remain in bound states.

With the energy relentlessly coming from its environment, these electrons can move to different orbitals, absorbing a portion of the star’s energy. When they do, they leave a characteristic signature in the star’s light spectrum: an absorption feature. When we look at the absorption lines of stars, they can tell us what elements they’re made of, what temperature they’re emitting at, and how quickly they’re moving, both rotationally and with respect to our motion.

The solar spectrum shows a significant number of features, each corresponding to absorption properties of a unique element in the periodic table or a molecule or ion with electrons bound to it. Absorption features are redshifted or blueshifted if the object moves towards or away from us.

Nigel A. Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF

The more accurately you can measure the wavelength of a particular absorption feature, the more accurately you can determine the star’s velocity relative to your line-of-sight. If the star you’re observing moves towards you, that light gets shifted towards shorter wavelengths: a blueshift. Similarly, if the star you’re monitoring is moving away from you, that light will be shifted towards longer wavelengths: a redshift.

This is simply the Doppler shift, which occurs for all waves. Whenever there’s relative motion between the source and the observer, the waves received will either be stretched towards longer or shorter wavelengths compared to what was emitted. This is true for sound waves when the ice cream truck goes by, and it’s equally true for light waves when we observe another star.

A light-emitting object moving relative to an observer will have the light that it emits appear shifted dependent on the location of an observer. Someone on the left will see the source moving away from it, and hence the light will be redshifted; someone to the right of the source will see it blueshifted, or shifted to higher frequencies, as the source moves towards it.

Wikimedia Commons user TxAlien

When the first detection of exoplanets around stars was announced, it came from an extraordinary application of this property of matter and light. If you had an isolated star that moved through space, the wavelength of these absorption lines would only change over long periods of time: as the star we were watching moved relative to our Sun in the galaxy.

But if the star weren’t isolated, but rather had planets orbiting it, those planets would cause the star to wobble in its orbit. As the planet moved in an ellipse around the star, the star would similarly move in a (much smaller) ellipse in time with the planet: keeping their mutual center-of-mass in the same place.

The radial velocity (or stellar wobble) method for finding exoplanets relies on measuring the motion of the parent star, as caused by the gravitational influence of its orbiting planets. Even though the planet itself may not be visible directly, their unmistakable influence on the star leaves a measurable signal behind in the periodic relative redshift and blueshift of the photons coming from it.

In a system with multiple planets, these patterns would simply superimpose themselves atop one another; there would be a separate signal for every planet you could identify. The strongest signals would come from the most massive planets, and the fastest signals — from the planets orbiting most closely to their stars — would be the easiest to identify.

These are the properties that the very first exoplanets had: the so-called “hot Jupiters” of the galaxy. They were the easiest to find because, with very large masses, they could change the motion of their stars by hundreds or even thousands of meters-per-second. Similarly, with short periods and close orbital distances, many cycles of sinusoidal motion could be revealed with only a few weeks or months of observations. Massive, inner worlds are the easiest to find.

A composite image of the first exoplanet ever directly imaged (red) and its brown dwarf parent star, as seen in the infrared. A true star would be much physically larger and higher in mass than the brown dwarf shown here, but the large physical separation, which corresponds to a large angular separation at distances of under a few hundred light years, means that the world’s greatest current observatories make imaging like this possible.

European Southern Observatory (ESO)

On the complete opposite end of the spectrum, some planets that are equal to or greater than Jupiter’s mass are extremely well-separated from their star: more distant than even Neptune is from the Sun. When you encounter a system such as this, the massive planet is so hot in its core that it can emit more infrared radiation than it reflects from the star it orbits.

With a large enough separation, telescopes like Hubble can resolve both the main star and its large planetary companion. These two locations — the inner solar system and the extreme outer solar system — were the only places where we had found planets up until the explosion of exoplanets brought about by NASA’s Kepler spacecraft. Until then, it was only high-mass planets, and only in the places where they aren’t found in our own Solar System.

Today, we know of over 4,000 confirmed exoplanets, with more than 2,500 of those found in the Kepler data. These planets range in size from larger than Jupiter to smaller than Earth. Yet because of the limitations on the size of Kepler and the duration of the mission, the majority of planets are very hot and close to their star, at small angular separations. TESS has the same issue with the first planets it’s discovering: they’re preferentially hot and in close orbits. Only through dedicated, long-period observations (or direct imaging) will we be able to detect planets with longer period (i.e., multi-year) orbits.

NASA/Ames Research Center/Jessie Dotson and Wendy Stenzel; missing Earth-like worlds by E. Siegel

Kepler brought about a revolution because it used an entirely different method: the transit method. When a planet passes in front of its parent star, relative to our line-of-sight, it blocks a tiny portion of the star’s light, revealing its presence to us. When the same planet transits its star multiple times, we can learn properties like its radius, orbital period, and the orbital distance from its star.

But this was limited, too. While it was capable of revealing very low-mass planets compared to the earlier (stellar wobble/radial velocity) method, the primary mission only lasted for three years. This meant that any planet that took longer than about a year to orbit its star couldn’t be seen by Kepler. Ditto for any planet that didn’t happen to block its star’s light from our perspective, which you’re less likely to get the farther away from the star you look.

The intermediate distance planets, at the distance of Jupiter and beyond, were still elusive.

The planets of the Solar System are difficult to detect using present technology. Inner planets that are aligned with the observer’s line-of-sight must be large and massive enough to produce an observable effect, while outer worlds require long-period monitoring to reveal their presence. Even then, they need enough mass so that the stellar wobble technique is effective enough to reveal them.

Space Telescope Science Institute, Graphics Dept.

That’s where a dedicated, long-period study of stars can come in to fill in that gap. A large team of scientists, led by Emily Rickman, conducted an enormous survey using the CORALIE spectrograph at La Silla observatory. They measured the light coming from a large number of stars within about 170 light-years on a nearly continuous basis, beginning in 1998.

By using the same instrument and leaving virtually no long-term gaps in the data, long-term, precise Doppler measurements finally became possible. A total of five brand new planets, one confirmation of a suggested planet, and three updated planets were announced in this latest study, bringing the total number of Jupiter-or-larger planets beyond the Jupiter-Sun distance up to 26. It shows us what we’d always hoped for: that our Solar System isn’t so unusual in the Universe; it’s just difficult to observe and detect planets like the ones we have.

While close-in planets are typically discoverable with stellar wobble or transit method observations, and extreme outer planets can be found with direct imaging, these in-between worlds require long-period monitoring that’s just beginning now. These newly-discovered worlds, down the line, may become excellent candidates for direct imaging as well.

E. L. Rickman et al., A&A accepted (2019), arXiv:1904.01573

Even with these latest results, however, we still aren’t sensitive to the worlds we actually have in our Solar System. While the periods of these new worlds range from 15 to 40 years, even the smallest one is nearly three times as massive as Jupiter. Until we develop more sensitive measurement capabilities and make those observations over decadal timescales, real-life Jupiters, Saturns, Uranuses and Neptunes will remain undetected.

Our view of the Universe will always be incomplete, as the techniques we develop will always be inherently biased to favor detections in one type of system. But the irreplaceable asset that will open up more of the Universe to us isn’t technique-based at all; it’s simply an increase in observing time. With longer and more sensitive observations of stars, closely tracking their motions, we can reveal lower-mass planets and worlds at greater distances.

This is true of both the stellar wobble/radial velocity method and also the transit method, which hopefully will reveal even smaller-mass worlds with longer periods. There is still so much to learn about the Universe, but every step we take brings us closer to understanding the ultimate truths about reality. Although we might have worried that our Solar System was in some way unusual, we now know one more way we’re not. Having gas giant worlds in the outer solar system may pose a challenge for detections, but those worlds are out there and relatively common. Perhaps, then, so are solar systems like our own.

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