Beyond Our Solar System

Exoplanets 101: Hopkins scientists explain how planets outside of our solar system are created and discovered—and whether life might exist on them.

In 1996, a buzz that swept through the field of astronomy left a college freshman intrigued.

The year before, a pair of Swiss astronomers had published findings suggesting for the first time in human history that there was a planet revolving around a distant star far beyond our solar system, in the constellation Pegasus. 

Their observations, which took scientists a year to fully embrace, led to an explosion in the hunt for planets other than the eight that circle our own Sun, and instead might be light-years away orbiting other stars in the Milky Way. 

Excitement grew that the discovery of new planets might offer answers to some of astronomy’s most fundamental questions, including how planets are created, their varying compositions, and even whether there are other worlds capable of harboring biological life.

“That was a big discovery,” says David Sing, the once awestruck college freshman who is now a Bloomberg Distinguished Professor in the Krieger School with joint appointments in the departments of Physics and Astronomy, and Earth and Planetary Sciences. “At that time people didn’t know exoplanets existed,” Sing explains. “You find the first one and, oh my goodness. It’s very much on the road to the discovery of other life. So it was a big deal.” 

David Sing

Using remote sensing techniques, we get to explore these unique worlds and environments and find out what planets across the galaxy are actually like.”

—David Sing

Today, some 25 years later, Hopkins scientists are leading the way in finding and learning more about what are now called exoplanets. By definition, a planet is something that orbits a star, has sufficient gravity to hold itself in a spherical shape, and has the gravitational power to clear other objects of similar size from its orbital path around its sun. Exoplanets do all of these things, but while orbiting stars far from our Sun.

Sing says he began pondering the vastness of the universe as a child growing up under the inky night sky in Montana, where the lack of light pollution reveals the vastness of the Milky Way. Today, he uses data collected from spectrometers—instruments that measure how light is influenced by elements and compounds the light interacts with—to study the chemistry and physics of the atmospheres of exoplanets. A planet’s atmosphere is our window into characterizing what these distant planets are like.   

“We still cannot predict what any particular exoplanet will look like in detail,” Sing says. “Even planets that are now known to be common throughout the galaxy, like super-Earths, have no analogy to any planets in our own solar system, so are totally new.”

Several scientists across Johns Hopkins are researching exoplanets, including three from the School of Arts and Sciences in addition to Sing: Kevin Schlaufman, who studies planet formationSarah Hörst, whose research focuses on planetary atmospheric hazes; and Jocelyne DiRuggiero, who examines the possibility of extraterrestrial life.  

A arge, yellow gas giant, and three smaller Neptune-like, super-earth and terrestrial planets.
Image: NASA/JPL-Caltech

What’s Out There? Exoplanet Types

Exoplanets, planets beyond our solar system, can make the planets closer to home look tame by comparison. “Hot Jupiters” are star-hugging, infernal worlds. “Super-Earths” are mysterious and more massive than Earth. Frozen planets—gas giants that make Jupiter look small—are rocky planets in Earth’s size range but in tight orbits around red dwarf stars.

How to Find a Planet 

Finding a planet that is in the grasp of a star some light-years away is no easy task. For one, the direct light from stars is so bright relative to the dim light of their relatively small, unlit satellites that an exoplanet likely would be lost in its glare. 

One method—the radial velocity technique—was developed as astronomers realized that the wavelength of light emitted by some stars shifts from blue to red and back again with periodic regularity. They suggested that any unseen planet orbiting the star would alternately pull the star toward and away from Earth with each pass, resulting in a blue-to-red-to-blue Doppler shift in the wavelength of the observed light. 

Other astronomers looked for the periodic dimming of a given star. If the dimming happened on a regular schedule, they reasoned, it could indicate that a planet was revolving around that star, and was blocking a tiny bit of the light emitted by its solar captor with each annual transit. 

Still other astronomers worked to perfect a lesser-used technique known as astrometry to infer the presence of exoplanets. Rather than studying Doppler shifts, astrometric analysis detects the telltale wobbling of planet-bearing stars by using precise measurements to predict the star’s path through the heavens, then observing slight deviations in its expected route. The deviation hints of the gravitational tug of an orbiting mass of significant size—a planet. 

But until the 1990s, the relative handful of astronomers pursuing these techniques largely toiled in obscurity, their work eclipsed in the popular imagination by scientists pursuing more fanciful topics, such as possible signs of the existence of intelligent life, or the method by which galaxies are formed. At the time, Sing was doing his doctoral work on another subject altogether—observing the cataclysmic dance performed by a pair of orbiting stars, as one steadily sucked the mass of the other into its white-hot interior. 

But speculation about the existence of exoplanets became provable in 1995, when Swiss astronomers Michel Mayor and Didier Queloz studied what appeared to be minute gravitational tugs on a distant star known as 51 Pegasi. They determined that 51 Pegasi’s wobble was the result of a gas giant planet (51 Pegasi b) similar to Jupiter, but whose orbit was 10 times closer than Mercury is to our Sun. The “b” means that this planet was the first discovered orbiting its parent star. The announcement of their discovery earned them a share of the 2019 Nobel Prize in Physics. 

A bright orange exoplanet 51 Pegasi b in a black sky
The exoplanet called 51 Pegasi b was discovered in October 1995. It is about half the size of Jupiter and orbits its star in about four days. The discovery helped launch a whole new field of exploration into exoplanets.
Image: NASA/JPL-Caltech

“Our understanding of how planets form has been greatly expanded by the enormous diversity of exoplanets in our galaxy,” says Kevin Schlaufman, an assistant professor in the Department of Physics and Astronomy. 

It took some time, however, before the discovery of the first exoplanet was fully appreciated. 

“[You would] think the  discovery would have been a big deal,” Schlaufman says. “But if you go back and look for articles the day after the announcement in the The New York TimesThe Wall Street Journal, it’s quite deep into the newspaper before you actually see them.” 

But before long, the discovery of 51 Pegasi b produced a shock wave of interest in finding new planets, studying their composition, and even searching for clues as to whether they might sustain life forms. Since that time, scientists have gone from knowing of no other planets beyond the eight in our solar system to having located more than 4,000 others. Evidence of several more were announced in the first two months of 2021 alone, including clues that one may be orbiting Alpha Centauri, one of the closest stars to our solar system.  

“Now the things that people are excited to work on are trying to understand whether or not there are signatures of life in some of the planetary systems we’ve discovered.” 

Kenneth Schlaufman

If you’re interested in answering the question of whether or not life as we know it on Earth can be found elsewhere in the galaxy, then you first need to establish that there are other planets out there. I think we’ve now established that.”

—Kevin Schlaufman

A Second Earth Waiting to Be Found?  

Far from being uniform in nature, exoplanets vary greatly in character. For example, “hot Jupiters,” the first exoplanets to be studied extensively, are gas giants that orbit their suns in as little as a few days, along paths that are much closer than Earth’s. “Ice giants” are more like our solar system’s Uranus, whose makeup consists of heavier elements such as water, methane, and ammonia. “Mini-Neptunes” are gaseous planets that are as much as 10 times more massive than Earth but closer in size. 

And there are “super-Earths,” planets that are rocky rather than gaseous, and whose orbits place them within the so-called “Goldilocks zone,” the distance from a sun where life-sustaining water can exist in liquid form. 

Perhaps the most intriguing discovery thus far came last year, when Kepler-1649c, an Earth-sized planet, was found in the Goldilocks zone of a sun some 300 light-years from here. 

“This intriguing, distant world gives us even greater hope that a second Earth lies among the stars, waiting to be found,” said Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate, when the planet’s discovery was announced in April 2020. 

In many ways, astronomers and physicists at Johns Hopkins University are at the front of the pack in the hunt for planets beyond our solar system. 

One of the most productive pieces of equipment ever devised for the study of deep space—the Hubble Space Telescope—has been operated for the entirety of the satellite’s 30-year Earth orbit from the Space Telescope Science Institute building at the edge of the Homewood campus in Baltimore. 

The scheduled Halloween Day launch of the James Webb Space Telescope will further expand research possibilities among teams at the STScI building. The Webb telescope has a mirror that is more than six times larger than Hubble’s, which allows it to see greater detail. Its spectrometers are designed to allow Webb to better study light’s infrared wavelengths, which give clues to the presence of water vapor. Scientists believe that will boost their ability to detect possible life-sustaining water in the atmospheres of distant planets. 

Before the COVID-19 pandemic forced people to work from home, the STScI building teemed with more than a hundred astronomers, physicists, engineers, and postdocs. Much of their work is done in collaboration with scientists in Hopkins’ physics and astronomy department, just across the street on San Martin Drive. 

Some of these researchers try to observe exoplanets directly using coronagraphs—instruments that block out the direct light of a star so that they can see nearby objects that otherwise would be hidden in the glare. 

Lightning in a Bottle 

It is the rare astronomer who has never pondered the question, “Is it possible there is a planet out there capable of sustaining life, and if so, what qualities would it have?” 

As of now, Earth remains the only planet known to be habitable in the universe. 

Earth orbits the Sun within a region that is called the habitable zone—a distance from its host star where a similarly sized planet could retain surface water in liquid form. It has an atmosphere with a specific mixture of gases—one being carbon dioxide—that traps enough heat so Earth is neither a frozen wasteland nor an arid inferno. It has abundant surface water. And its mass is in a range that encourages fissures in its tectonic plates, which allow gases from the planet’s interior to replenish its atmosphere, which also shields potential life on the surface from lethal levels of solar radiation. 

Krieger School Associate Professor Sarah Hörst’s involvement in the study of exoplanets extrapolates from her research on Titan, an icy, frozen moon of Saturn. It is massive enough to also have its own atmosphere and temperate enough to harbor below its surface the magic ingredient of life as we know it: liquid water. And when she’s not mountain biking, doing triathlons, or trying to coax terrestrial life from her home garden, she’s in her Olin Hall laboratory, trying to catch 3 billion-year-old lightning in a bottle. 

Titan’s atmosphere is considered to be favorable for producing the kind of organic haze believed to have existed on Earth during the Archean eon some 3.8 to 2.5 billion years ago. That and the presence of oxygen‐bearing molecules have astronomers curious about Titan’s atmosphere as possibly harboring molecules that could lead to biological activity. 

“Atmospheres in our solar system have some differences,” says Hörst. “We have CO2 atmospheres with Venus and Mars, we have nitrogen atmospheres with Earth and Titan, but by no means does our solar system span what is actually possible in terms of atmospheric temperatures, pressures, and compositions of planetary atmospheres.” 

Sarah Hörst

One of the reasons we have been excited about exoplanets is that in addition to helping us learn about specific exoplanets, it also helps us build a framework for an understanding of how planetary atmospheres work. Because we’re trying to figure out how planets work.”

—Sarah Hörst

Hörst’s work mainly involves trying to recreate the combination of gases, atmospheric pressure, and temperature believed to characterize the atmosphere of a given exoplanet. She then exposes these simulated atmospheres to sources of energy that planetary atmospheres typically experience in nature, such as powerful ultraviolet radiation from starlight, and plasma—fields of ionized gas that can occur when lightning strips electrons from atoms, making those atoms highly reactive. 

Hörst’s experiments have produced hundreds of carbon-nitrogen compounds, which suggest that exoplanet atmospheres are capable of producing the building blocks of biological activity. Her documentation of the optical properties of the compounds produced in simulated exoplanet atmospheres is expected to boost the value of the upcoming James Webb Space Telescope, whose expanded instrumentation will give astronomers more precise observations of the chemical makeup and physical properties of exoplanet atmospheres. If, for example,  researchers find signs of oxygen in a distant atmosphere, her experiments will help determine whether its presence can be ascribed to ordinary chemical and geologic processes, or instead signals the presence of life. 

Life Inside of Rocks 

But searching the heavens for an Earth twin—one with freshwater lakes, vast grasslands, or perhaps forests teeming with wildlife—may be asking a bit much. 

In fact, Jocelyne DiRuggiero does most of her research on the possibility of extraterrestrial life by looking down rather than up. 

DiRuggiero, a biologist and associate research professor at the Krieger School, scours the surface of the Atacama Desert in northern Chile, driving around in a four-wheel drive vehicle and hiking to find inhabited rocks. The Atacama’s high plateau is one of Earth’s most hostile environments. Parts of it typically go for a decade or more without rain, and are strewn with translucent rocks. 

An ancient form of life called cyanobacteria finds the interior of the boulders to be a pleasant nursery. The bacteria make use of water that reaches the rocks’ interior via tiny fissures. The translucent rocks transmit enough light for the bacteria to photosynthesize carbohydrates, but also shield the bacterial colonies from the toxic intensity of the desert sun.  

Voila. Life inside of rocks. 

Watch a video about Jocelyne DiRuggiero’s travels to the Atacama Desert to break open rocks to study extremophiles.

The microbiota found in these extreme environments typified the life forms that first emerged about 750 million years after Earth first formed some 4.6 billion years ago. DiRuggiero’s findings have helped guide NASA engineers to conceptualize the sampling tools carried by the rover that landed earlier this year on Mars. And they give astronomers clues as to what to look for as they examine exoplanetary atmospheric and other observations for indications that life might exist on planets where extreme temperatures, scorching solar radiation, or the scarcity of liquid water are the norm. 

“When you start to probe the limits for life on Earth, you start wondering if it is just on Earth or is there life out there,” DiRuggiero says. “And how learning about the limits on life here might inform our search for life elsewhere.”  

Visions of Extraterrestrial Worlds 

The possibility that there are planets with sentient life existing beyond human contact—a concept known as cosmic pluralism—dates back at least to the time of the Greek philosopher Democritus (c. 460 to c. 370 B.C.), who also proposed that matter was composed of atoms. 

In the late 1500s, the Italian theorist Giordano Bruno correctly speculated that the stars were distant suns much like our own. He said they might be surrounded by life-harboring planets of their own. 

In 1610, Galileo Galilei was the first person to make telescopic observations of a planet in our solar system when he pointed his newly constructed telescope at Venus to discover its phases and at Jupiter to discover its four largest moons. 

More than a century later, in 1781, the British astronomer Sir William Herschel discovered Uranus while making a survey of the night sky. That led him to speculate whether life might be found there. 

That same year, Emmanuel Kant published his Critique of Pure Reason, which includes the passage, “I say that it is not merely an opinion but a strong belief (on the correctness of which I would wager many advantages in life) that there are also inhabitants of other worlds.” 

Visions of extraterrestrial life animated 20th-century science fiction. Superman, perhaps science fiction’s first superstar, was imagined to have come from a doomed planet that orbited a dying sun 27 light-years from Earth. And the Star Wars saga, science fiction’s most financially successful franchise, has the Jedi Master Yoda seek refuge in Dagobah, a mist-shrouded swamp planet reminiscent of Earth’s Carboniferous Period.