Six questions in astronomy and astrophysics

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At the Earth and Planets Laboratory (EPL), the astronomy and astrophysics team explores space to find distant planets and understand their (and our!) origins.
This illustration shows a star surrounded by a protoplanetary disk. Image credit: NASA/JPL-Caltech

At the Earth and Planets Laboratory (EPL), the astronomy and astrophysics team explores space to find distant planets and understand their (and our!) origins. Below we’ve distilled this work into our top six research questions (with a couple of extras sprinkled in.) Each question is complex and connected to the others in a way that requires EPL scientists to collaborate across research interests to answer. In that way, the scientists at Carnegie are perfectly situated to get at some of our universe’s biggest questions.

1) How do planetary systems form?
2) How many and what kinds of exoplanets are there?

3) Why are there so many types of planetary systems?
4) How do gas giant planets form?
5) What makes a planet habitable?
6) What lies at the fringes of our Solar System?

1. How do planetary systems form?

Star surrounded by protoplanetary disk.jpg
This illustration shows a star surrounded by a protoplanetary disk. Image credit: NASA/JPL-Caltech

Protoplanetary disks are rotating frisbees of gas and dust that are created when stars form. The dust and gas ultimately coalesce to form planets that settle into orbits around their star—creating what we know as a planetary system. Understanding how these early systems go from disk to planets is essential to understanding our own origin and the true scope of our place in the universe. 

What size planets form and what they are made of depends on how the materials in disks get distributed over time, what the balance of different elements and ices is within the gas and dust, and how fast the disks dissipate.

Observing Disks and Planet Development

Observational astrophysicist Alycia Weinberger directly observes protoplanetary disks in order to understand how planets form. Weinberger uses a variety of observational techniques and facilities to tease apart disk development.

TW Disk.jpg
The star TW Hydrae is between 5 and 10 million years old, and should be in the final throes of planet formation before its disk dissipates. This image from the Hubble Space Telescope (HST) shows a deficit of material, i.e. a partial gap, at about 80 AU from the star. The gap is consistent with what would be created by a 6-30 Earth mass planet within it. Weinberger and her colleagues have imaged TW Hya at seven wavelengths with HST to understand not only whether planets have formed, but also what conditions can result in planet formation and what chemical constituents are available for new planets.

For example, with the Hubble Space Telescope, Weinberger images the smallest dust grains in disks at different wavelengths to examine the composition and evolutionary stage of the disk. With Carnegie’s Magellan Telescopes, she looks for where and how abundant ices and organic compounds exist throughout disk evolution. With the Large Binocular Telescope Interferometer, she is searching for faint remnant disks, such as the Solar System’s Zodiacal cloud, which betray the presence of planets and asteroids that might otherwise not be known.

Weinberger said, “My primary scientific goal is to understand the composition and evolution of planet-forming material in circumstellar disks. I think the physics of planet formation is the most exciting aspect of the field of exoplanets.”

Modelling System Development

It takes up to hundreds of millions of years for a planetary system to form. Constructing mathematical models of Solar System creation is another way scientists fill in the gaps and test out new theories. Theoretical astrophysicist Alan Boss creates mathematical models of disks using the information we know from observations like those done by Weinberger.

For instance, in a 2020 publication, Boss, Conel Alexander, and colleague Morris Podolak (Tel Aviv U.) explore the thermal evolution of particles in these disks. Boss’ 3D models explore how the crystalline silicates—materials that make up much of the Earth’s crust and that are observed in protoplanetary disks—could have been formed by thermal “cooking” close to their protostar and then transported outward to cooler regions of the disk.

2. How many and what kinds of exoplanets are out there?

Gas Giant and sun.jpg
Artist’s impression of a gas-giant exoplanet passing by its host star. Image Credit: ESA, NASA, G. Tinetti (University College London, UK & ESA) and M. Kornmesser (ESA/Hubble)

Ever since astronomer Paul Butler confirmed the existence of the first extrasolar planet orbiting a sun-like in 1995—a “hot Jupiter” called 51 peg b—exoplanetary science has been on the rise! Since that first discovery, scientists have found more than 4,300 planets orbiting stars other than our own. Today one of the main focuses of scientists at the Earth and Planets Laboratory remains the discovery of these planets—particularly the search for Earth-like planets around sun-like stars. 

Transit Method vs. Doppler Velocity

Transit Method:

A NASA animation of the transit method of exoplanet discovery.  The line on the left of the figure denotes the amount of light reaching Earth, which dims as the planet passes in front of its star. Credit: NASA

There are two main ways scientists at Carnegie find exoplanets. In the transit method, a planet passes between a star and our telescope so astronomers measure a dip in the light. In the Doppler velocity method—also known as the radial velocity method—the push and pull between a star and its orbiting planet causes the star to wobble slightly on its axis.

Astronomers can then measure the change in velocity of the star as it moves toward and away from our telescopes. 

The Search for Exoplanets

Since the mid-1980’s Butler’s focus has been producing and improving the precision of Doppler velocity measurements of nearby stars. In the mid-1980s Doppler velocity precision was stalled at 300 m/s. (In contrast, Jupiter induces a 10 m/s Doppler velocity variation on the Sun.) Starting in chemistry labs at San Francisco State University, Butler designed and built the first precision velocity Iodine absorption cell in 1987. It took 8 years to improve the Doppler velocity precision from 300 m/s to 3 m/s.

Now, a team that includes EPL’s Butler and Johanna Teske (along with Stephen Schectman and Jeff Crane at the Carnegie Observatories) are working to improve that precision even further to 1m/s and beyond. Teske is also leading the effort to understand the planetary systems that are emerging from the NASA TESS mission, which uses the transit method to observe huge swaths of sky at once.

Astrometry: a third method? 

Alan Boss, Alycia Weinberger, and Ian Thompson (from Carnegie Observatories) have been running the Carnegie Astrometric Planet Search (CAPS) program, which searches for extrasolar planets by the astrometric method. Similar to radial velocity, astronomers using the astrometry method detect a planet's presence indirectly through the wobble of the host star around the center of mass of the system. However, instead of measuring the changes in velocity, astrometry relies on precise measurements of the star’s position. 

A NASA animation of the astrometry method of exoplanet discovery. Credit: NASA

No exoplanets have been discovered orbiting single stars using this method so far, but once perfected, it could allow astronomers to detect exoplanets that are currently undetectable because of their orbit orientations or masses. With over thirteen years of CAPSCam data, the team is beginning to see likely true astrometric wobbles.

3. Why are there so many different types of planetary systems?

The image shows an overhead view of orbital positions of the planets in systems with multiple transiting planets discovered by NASA's Kepler mission. Credit: NASA Ames/Dan Fabrycky, UC Santa Cruz

The Solar System is only one example of a planetary system. Thousands of other systems have been discovered, and most look very different than our own. Many other systems contain planets somewhat larger than Earth orbiting close to their stars where they would be too hot to support life. Giant planets like Jupiter are also quite common, but many have orbits that would prevent the existence of an Earth-like planet in a habitable orbit.

Understanding what controls the architecture of a planetary system and why observed systems are so diverse are key questions in the search for other planets like Earth. Astrophysicist John Chambers addresses these questions using computer simulations of planet formation. 

Says Chambers, “An important caveat is that the realism of these models needs to be tested by comparing the output with the observed orbits and masses of real planets.”

In a 2018 study, Chambers carried out a very large suite of planet formation simulations using a model that was gradually trained to match the known distribution of exoplanets—planets orbiting other stars. This study helped to constrain some important aspects of planet formation that were previously poorly known.

How do sub-Neptunes and super-Earths form?

These planets are the most common in the Galaxy, and yet we don't have one in our Solar System. To understand why we first need to know what these planets are made of so we can attempt to pin down how they are formed.

To do this, astronomer Johanna Teske takes mass and radius measurements of planets to estimate their compositions. She is also starting to work on how to measure and interpret the atmospheric compositions of these planets, and what their atmospheres can tell us about the interior composition (e.g., gasses released into the atmosphere from inside of the planet). For example, in a 2020 publication, Teske characterized a sub-Neptune found by TESS. 

Like much of the work done on campus, this research is collaborative. She aims to incorporate the new data coming out of the high pressure and temperature experiments that are done in other labs on campus, which mimic the conditions of planetary interiors. This will allow her to more accurately understand what’s going on inside of these planets and better interpret her observations.

4. How do gas giant planets form?

Jupter Nasa Flyby
An image of Jupiter from NASA’s JUNO spacecraft. NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill

Massive and gassy, Jupiter and Saturn are planets most people know. But perhaps what many people don't know is that similar gas giants are quite common in other planetary systems. Scientists are currently studying how these planets form, and why they can be found in locations and orbits much different than in our system. That mystery is one of the toughest problems in planetary science.

Core Accretion vs. Disk Instability

There are two main ways we think planets form in these protoplanetary disks. The core accretion mechanism holds that a gas-giant first forms a large, mostly rocky, core with enough mass and hence gravity to pull in gas from the surrounding disk to form a thick atmosphere, eventually becoming huge gas worlds. The competing idea is the disk instability method, in which uneven swaths of dust and gas from the protoplanetary disks are big enough to create their own gravitational centers and condense in on themselves into protoplanets. It’s possible that planet formation is some mix of the two.

In 2017, modeling the core accretion method, John Chambers found that planets somewhat smaller than the Moon are large enough to capture a thin atmosphere. The atmosphere grows thicker and hotter as the planet gets bigger. Once the planet is similar to Mars in size, it can form an ocean. At larger masses, the planet's ocean boils and the atmosphere becomes a dense mixture of steam and hydrogen and helium. When a planet reaches a few times the mass of Earth, the atmosphere will grow rapidly, faster than the solid part of the planet, eventually forming a gas giant planet like Jupiter.

On the other hand, Alan Boss is currently working to apply the disk instability model to the problem of exoplanet populations. Observations have shown that gas giant exoplanets can orbit at large distances from their stars, where the competing mechanism of slow core accretion has trouble forming gas giant exoplanets. Boss has shown that disk instability has no such problem, as it is a rapid means for forming protoplanets during the short life of planet-forming disks.

Says Boss, “Disk instability (DI) can form gas giants even in short-lived disks, provided they are massive and cool enough, and so I continue to investigate the extent to which disk instability might help solve this problem in exoplanet population synthesis studies.”

Is This How Baby Gas Giants Are Born?
This simulation of the disk of gas and dust surrounding a young star shows dense clumps (denoted in redder colors) forming in the disk. According to the proposed disk instability method of planet formation, they will contract and coalesce into a baby gas giant planet. Credit: Alan Boss

Do metal-rich stars make metal-rich planets? 

The relationship between the compositions of gas giants and their host stars is key to understanding planet formation. We know that close-in (hot) giant planets are more often found around stars with high iron abundances/metal abundances in general. This led to the obvious question: Do metal-rich stars also make metal-rich planets?

Teske and her colleagues set out to investigate this question in their 2019 paper. Given a planet’s mass, astronomers expect planets to have certain metal composition. The difference between the expected metal composition and its actual composition is called “residual metallicity.” Before Teske’s study, astronomers assumed a star’s metal composition would be directly related to this residual metallicity, which would suggest that a star’s composition could be used to predict a planet’s metal composition.

To Teske’s surprise, her team found no clear relationship between the metal composition of a star and giant planets’ residual metallicity.  However, they did find a potential connection between residual planet metals and their star’s volatile-to-refractory element ratios—the ratio between carbon and oxygen (volatiles) compared to other elements (like Si, Mg, Fe, and Ni). Teske’s team introduced the idea that the stellar volatile-to-refractory ratio could be a marker of when to expect more metal-rich planets, but further work is needed to confirm this tentative finding.

We don’t know everything about how metals are distributed through giant planets, so additional observations of giant planet atmospheric composition with the upcoming James Webb Space Telescope will complement Teske’s work mapping the connection between star and planet compositions.

5. What determines if a planet is habitable?

Planets are the only place where we know that life can thrive. However, not all planets are habitable. There are many factors that could affect habitability including composition, surface temperature, presence of a magnetic field, and stellar activity.

Determining what makes a planet habitable and finding habitable planets is the ultimate goal of the Carnegie Planets interdisciplinary research project. Much of the astronomy and astrophysics work (as well as interdisciplinary studies across the EPL campus) is dedicated to exploring the question of planetary habitability. 

One example of this type of research is John Chambers’ work to understand how a planet may develop a stable climate on geologic timescales. 

The climates of Earth-like extrasolar planets cannot be directly observed at present, but they can be studied using numerical climate models. On Earth, our climate is controlled primarily by the amount of solar radiation it receives and captures, greenhouse gases, and—on very long timescales—by the exchange of greenhouse gases between the atmosphere and the interior of the planet. Stabilizing feedback keeps the average surface temperature suitable for liquid water to exist—a key factor in whether a planet can host life

Whether other planets behave in the same way depends on how greenhouse gases evolve on these planets as well as the amount and type of light the planet receives from its star. In a recent study, John Chambers used a climate model to examine chemical reactions on the seafloor that can play an important role in controlling climate.  

How many Earth-like planets are there?

A recent NASA study published in The Astronomical Journal announced that between 37% and 88% of sun-like stars might have rocky worlds orbiting in their habitable zones, defined as the distance from their star that would allow a surface temperature within the range of liquid water. The team of scientists, which includes Alan Boss, looked at Kepler transit data from over 80,000 stars and determined that there is roughly one Earth-like planet for every two sun-like stars in our galactic neighborhood, a higher frequency than previously imagined.  

6. What lies at the fringes of our Solar System?

Earlier in this article, we discussed how and why we study distant planetary systems, but the easiest planetary system to study is the one we're in! We are still learning new things about our own Solar System which can help us piece together the puzzle of how planetary systems and habitable planets form. One of the biggest mysteries in our Solar System is what objects exist past Pluto in the far, dark reaches of our Sun's gravitational pull. 

The Kuiper Belt, where Pluto lives, is a region of comet-like objects just beyond Neptune. This belt of objects has an outer edge, which we are only now able to explore in detail. For the past few years, astronomer Scott Sheppard and his colleagues have been performing the largest and deepest survey ever attempted to search for distant Solar System objects.

The ongoing search has discovered the object with the most distant orbit known in our Solar System and several of the largest known objects after the major planets. These extremely distant objects are strangely grouped closely together in space, which suggests a yet unobserved planet more massive than the Earthalso known as Planet X— is shepherding them into these similar orbits. (During these searches of the distant Solar System, Sheppard has also discovered near-Earth asteroids and quite a few moons of Jupiter and Saturn.)

The orbits of these objects also give us clues into our Solar System’s early years, including the distinct impression that there was once a third ice-giant planet in our outer Solar System. This ice-giant was likely flung far past Pluto during  Jupiter and Saturn’s gravitational dance to find their current stable orbits. Ultimately, this information helps us understand how this and other Solar Systems may form. 

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