Carnegie scientists answer your astronomy questions

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Thursday, March 25, 2021 


You asked and we answered! Earlier this month our astronomers asked our digital community to submit questions that were out of this world!  We received many great questions ranging from the origins of life to the existence of dark matter. 

Below you will find a list of the most common questions we received along with answers from the scientific experts who work at the forefront of that topic. We also included several questions we received from our Neighborhood Lecture Series, which have been answered by Alycia Weinberger.


The story of the emergence of life on Earth seems to be filled with many highly specific accidents (like the formation of the Moon) and events (like mass extinctions). How much variation could there be in the history of a planet for it to develop life?

Alycia Weinberger (Staff Scientist, Earth and Planets Laboratory): The short answer is that we don't know because we only have one example right now of a planet we know developed life. The interdisciplinary work we're doing with the Carnegie Planets project is dedicated to answering exactly these types of questions.

The process of the origin of life on Earth is still a mystery, but life apparently arose pretty quickly and persisted. That's not to say that all life survived (sorry dinosaurs), but after the Earth cooled following the Moon-forming impact, i.e. from a few hundred million years after Earth formation until now, Earth has been stable enough to support life. Many of us at EPL are interested in the question of what characteristics of a planet are necessary to sustain life. For example, Earth's composition and temperature let us have a magnetic field and plate tectonics.

Now that we know that planets around other stars are common, we can try to figure out if they followed the same basic formation pathway that Earth did.

What are the names of Saturn’s 20 new moons?

Scott Sheppard (Staff Scientist, Earth and Planets Laboratory): Here’s where we are with this Saturn’s Moon competition.

The International Astronomical Union naming committee would like the moons to be observed one more time before they give any official names because the discovery observations Sheppard used were from several years ago. This unexpected twist to our normal moon finding and naming workflow may mean that the moons (and thus their names) won’t be officially confirmed until they are observed again. This, unfortunately, may take a long time.

We don’t want to give any moon namers false hope, so we won't release any of the names until they are confirmed. However, we do want to acknowledge and thank all of the people who submitted moon names. We have a really great list that we will be working from as the moons are confirmed. It just may not be something we can release all at once as we expected.

Vera Rubin and Kent Ford opened the modern era of Dark Matter investigation in the late 1970s. The composition of dark matter still seems to be mostly a matter of speculation. What have we learned in the last 40 years? What experiments or observations are most promising to add to our knowledge?

Andrew Benson (Staff Scientist, Carnegie Observatories): We've actually learned a lot! Perhaps most importantly, Rubin and Ford's conclusion that dark matter is ubiquitous in the universe has been confirmed over and over again—there's really no other way to explain the vast amount of data that now supports the idea of dark matter. This includes not only the galaxy rotation curves and cluster galaxy motions that originally suggested dark matter but also the high precision measurements of the Cosmic Microwave Background radiation that are in excellent agreement with expectations from dark matter theory.

But, we still don't know what dark matter consists of. Work in the 1980s and ‘90s proved that dark matter couldn't be a very light particle, such as the neutrino (if it were, it would be impossible for galaxies to form). Observations that could reveal more about the nature of dark matter have to look for its subtle gravitational influence on light emitted by other galaxies or stars (since dark matter itself doesn't emit any light that we could observe through telescopes), and, it turns out, that the most interesting measurements to make are for the smallest dark matter structures - making the measurements even more difficult.

There are some very promising approaches though. For example, looking at distant galaxies whose light has been split into multiple images through gravitational lensing gives us a "dark matter microscope" which allows us to detect the presence, or absence, of tiny dark matter structures. Counting up the number of very faint galaxies - which can only form if there are small dark matter structures present to hold them together - can also help us learn more about the properties of dark matter. The next generation of ground-based and space-based telescopes (JWST, Roman Observatory, and the aptly-named Rubin Observatory) should provide us with invaluable new measurements in these areas - so there is a real opportunity to push our understanding of dark matter forward.

Note: The DOE Office of High Energy Physics and the NSF Physics Division fund experiments that are trying to directly detect dark matter particles. You can read more about those at: https://science.osti.gov/hep/Community-Resources/Next-Generation-of-Dire...

How far is the closest star?

Johanna Teske (Staff Scientist, Earth and Planets Laboratory): The closest star to the Sun is actually part of a triple star system! The dimmest and coolest star, Proxima Centauri, is closest to us at about 1.3 “parsecs”, where a parsec is an astronomical distance measurement equivalent to about 3.26 light-years. That means it takes the light from Proxima Centauri about 1.3 x 3.26 = 4.24 years to reach us. That’s about 25 trillion miles away! The other two stars in the triple system, Alpha Centauri A and B, are solar-type stars, and about 4.35 light-years away. Alycia Weinberger is part of a team of astronomers who have been studying the variability of Proxima Centauri using different wavelengths of light, which is particularly interesting because we know there is a planet around this star, which Carnegie EPL’s Paul Butler helped to detect!

Why did gas giants form in the outer solar system?

Alan Boss (Staff Scientist, Earth and Planets Laboratory): There are two mechanisms for forming gas giant planets, i.e., planets composed primarily of hydrogen gas. Both mechanisms work best in the colder regions of the rotating disk of gas and dust that formed our solar system, that is, at distances beyond about the present-day orbit of Jupiter. The “top-down” mechanism, disk instability, requires cold gas, in order for the gas to form of spiral arms and clumps as a result of its own gravity. The “bottom-up” mechanism, core accretion, works best where icy solids can form, providing more building blocks for assembling the ice/rock cores that are the first step in the core accretion process.

Note: Alan Boss’ book The Crowded Universe has even more information about this topic and more. Definitely worth a read!

How did the Moon form?

Matt Clement (Postdoctoral Fellow, Earth and Planets Laboratory): We think the Moon formed as a direct consequence of the terrestrial planet formation process itself. The terrestrial planets grew from a sea of smaller objects through a series of collisions that slowly consolidated the distribution of objects of various sizes into the modern four terrestrial planets: Mercury, Venus, Earth, and Mars.

The Moon is thought to have formed as a side-effect of the final massive accretion event on Earth; in a sense pulling itself together from the disrupted material flung off of the young Earth during the impact. There are multiple lines of reasoning pointing towards this hypothesis. In particular, the Earth and Moon’s chemical similarities and the total system angular momentum strongly suggest that the Moon was formed after an impact on Earth, rather than the capture of an independent body. Interestingly, we would naively expect Venus’ formation to have concluded with a similar, Moon-forming impact. Therefore, the fact that Venus has no Moon might imply that the final objects it accreted were much smaller than those accreted by Earth.

How do you figure out the compositions of planets and stars when they are so far away?

Johanna Teske (Staff Scientist, Earth and Planets Laboratory): Almost everything we learn about far-away objects like other planets and stars comes from observing how their light behaves. In the case of stars, we can observe their light as it’s filtered through their atmospheres (called the photosphere), which selectively absorb certain colors or wavelengths of light depending on the composition. Each element and molecule has a different chemical “fingerprint” that we can use to distinguish how much of each of them are present in a given gas. We measure these chemical fingerprints using astronomical instruments called spectrographs, which disperse the star’s light into its constituent colors like a prism does with sunlight.

There are a few different ways we try to figure out the compositions of planets. One is very indirect—we measure the planet’s mass and radius and calculate the average density of the planet, and compare that density to different models of composition based on different combinations of materials like rock and ice. (Measuring the masses and radii of planets is itself usually an indirect observation, too, based on the influence the planet has on its host star’s light!)

Another way is similar to the idea above for the star -- if a planet has an atmosphere, as it passes in front of its host star, some of the star’s light can filter through the planet’s atmosphere. If the atmosphere has, for example, water or carbon dioxide, we can see the “chemical fingerprint” of those molecules when they absorb some of the star’s light.

Questions from the Neighborhood Lecture

On March 16, 2021, observational astrophysicist Alycia Weinberger presented Come not between a dragon and its wrath, a tale of small stars and their planets as part of our Neighborhood Lecture Series. During her talk, Weinberger explained how the surprisingly intense stellar flares from small stars like Proxima Centauri might be effectively burning away any atmospheres on their orbiting planets.

Below, Weinberger answers questions from the audience about atmospheres, habitability, solar flares, and more.

Since all these planets are so close to the stars, do you expect them to be phase-locked, and does this have any implications for the behavior or evolution of the atmosphere or the geodynamo/magnetic field?

Tidal locking, where one side of the planet always faces the star, or where the planet rotates extremely slowly, such as Mercury rotating 3 times for every 2 times it goes around the Sun, has many potential implications for habitability.

We do expect that planets close to their stars, such as Proxima b, would be tidally locked. One thing that the process of tidal locking does is heat the planet. EPL scientist Peter Driscoll showed that this heating, somewhat counterintuitively, can help a planet form a long-lived magnetic field. Magnetic fields then can help shield a planet from high-energy radiation from the star that can be harmful or even deadly to life. Tidal locking could be very bad for an atmosphere. If one side of the planet is extremely cold, the atmosphere can freeze out there, and the atmosphere would flow from the warm side to the cold side until it was all completely frozen. However, it is also possible that a thick atmosphere could stay warm enough not to freeze entirely. We need to look for atmospheres and find out!

Can we measure the depth of the atmosphere that we detect? What gases can we detect besides H2 He? CO2? I am assuming O3 hasn't been found yet?

We hope to be able to say how much of each gas is in an atmosphere, not just that it is present. All elements and molecules have spectroscopic signatures, so the amount of each wavelength (color) of light absorbed or emitted by an atmosphere depends on how much of the element or molecule is there as well as temperature and gas motions from winds or atmospheric escape. By modeling the spectrum of a planet, astronomers can infer both the temperature structure of an atmosphere and its composition.

Right now, water vapor is the gas we can most easily detect, and it has been found in at least one sub-Neptune-sized planet that gets about the same amount of energy from its star as the Earth gets from the Sun (the planet is called K2-18b). Other gases that we can look for are methane, carbon monoxide, carbon dioxide, and ammonia as well as molecular oxygen and ozone (but so far we have not found evidence of O3 in an exoplanet atmosphere).

In giant planets, sodium and potassium are seen. Molecular hydrogen is not detected but can be inferred from how puffed up an atmosphere is. For very hot gas giant planets, so hot that rock would be vaporized, we can look for gaseous molecules only found at high temperatures such as SiO and TiO, ionized atomic species like silicon, iron, titanium, calcium, and magnesium, as well as evidence of escaping atomic hydrogen and helium. If you’re really interested in extreme atmospheric detections, check out KELT-9b, which is an exoplanet that is as hot as a star and has elements like scandium and yttrium detected in its atmosphere!

Is there existing technology to look for amino acids in these distant exoplanets?

Our first goal is to detect common gases, such as water, in the atmospheres on small exoplanets. Our second goal is to detect a whole suite of gases, so we can understand the atmospheric chemistry. Detecting something as rare and complex as an amino acid will be extremely difficult. One observation that might be possible eventually is to detect the circular polarization of light that can be caused by complex molecules.

Is it possible for a planet to have enough ozone or some other gas in its atmosphere to protect life from the radiation from being close to a small star?

Yes, it is possible. We just don’t know enough yet about how planets form and retain their atmospheres. I wouldn’t want to say that anything is impossible at this point. We have to go and look!

This is pretty basic, but why is it always a disc and not a globular formation around the stars? Always rotation causing it?

Yes, rotation is responsible for why the gas and dust settle into a disk rather than sit in a spherical geometry. It’s the same reason that pizza makers spin the dough to form the crust. Conservation of angular momentum causes the gas to flatten, although we often see warps and gaps in disks that we think are related to the formation of planets.

If there is complex life on planets around small stars, do you expect it would live in an ocean under the ice to protect it from the small star’s radiation? Do you think a technological civilization could develop underwater?

There are many places on Earth that are inhabited now, but we don’t know if life could have formed in those places to begin with. As a technological civilization, we might be able to figure out how to live under ice, but we don’t know if a technological civilization could develop underwater, or if there are other technological civilizations to begin with. The origin of life is a big mystery, so we don’t know if it could happen, even on Earth, entirely underwater or if it needs a surface-water interface to succeed.

Can the distance to stars within 10 parsecs be determined by parallax or is some other method used?

Parallax is the only direct method for measuring the distances to stars. Right now the European Space Agency mission called Gaia is measuring parallaxes to about 1.5 billion stars. The brighter the star, the further away Gaia can measure its distance. Gaia can easily measure some stars out to 7000 pc.

A recent paper estimated that it had measured the distances to more than 90% of all the small stars (M-type stars) within 100 pc. If we can’t or haven’t measured the parallax to a star, we can still estimate its distance by comparing its brightness to other stars of the same temperature that we have measured distances to.

So, the young, angry red stars are not the same as the old, red dwarf stars?

Young, angry red stars will become old, red dwarf stars. The younger the red dwarf, the more ultraviolet and X-ray light it emits and the more flares it has.

Why does a small red star put out a lot of UV radiation?

The UV is mainly from flares, not all of which have to be big. Flares are short events caused by energy released when magnetic fields on the star get twisted and then break and reconnect.

Do we have any evidence for exoplanets around Alpha Centauri (A and B) as well?

People are actively looking for planets around Alpha Cen A and B as well as Proxima. The primary method being used is radial velocities, but because the stars are so close to the Sun, people are also attempting to image around the stars looking for planets. Alpha Cen A and B are a fairly close binary, orbiting only 24 AU (astronomical unit, the distance between the Sun and the Earth) from each other, which we think makes the system not as hospitable a place for planets to form as in single stars or wider binary systems. Nevertheless, there are stable places for planets in the system, and it’s certainly worth looking further.

There was a planet candidate announced around Alpha Cen B back in 2012, but it’s now thought that the signal was likely caused by limitations in the data combined with stellar activity.

How do plate tectonics make a planet friendlier to life?

One way that plate tectonics makes a planet friendlier to life is by providing a way to keep the climate of a planet stable. On Earth, we have what’s called the carbonate-silicate-weathering cycle. When the Earth warms, carbon dioxide gets removed from the atmosphere when chemical reactions between carbon dioxide, water, and rock speed up. These reactions form carbonate that then runs off into the oceans and is buried; then plate tectonics drags the carbon molecules down into the Earth’s mantle.

When the Earth cools, these reactions slow, and that carbon dragged in by plate tectonics gets emitted again, as carbon dioxide from volcanoes. Carbon dioxide is a greenhouse gas that causes Earth to warm if it builds up in the atmosphere. So, this cycle balances carbon dioxide produced with carbon dioxide removed and keeps the temperature steady over geologic times. Humans can still change the greenhouse effect over short times.

Curious about the role of a big moon in stabilizing a planet's orientation, seasons, and if we can even say if planets like Prox b have nice big moons.

Moons are another big unknown. We think our own formed in a giant collision of a planetary embryo with Earth. We also think giant collisions are a natural part of the planet formation process. But how common moon formation is, we don’t know. Nor have any moons been found around exoplanets, yet. In principle, they can be found by their transit signature, as the planet and moon pass between us and their star.

Our Moon is very helpful in stabilizing the Earth’s orientation, which helps us have a long-term stable climate. Since the origin of life is a mystery, we can’t say if that stabilization is necessary for the development of life.

What is the periodicity of flares on our sun, and is similar across other stars?

Our sun has an 11-year cycle in its activity, including flares and sunspots. Right now we are coming out of Solar minimum and should reach Solar Maximum in mid-2025. Other stars, including red dwarfs, appear to have activity cycles as well. The duration can differ. Red dwarfs seem to have shorter cycles, on average more like 6-7 years.

What particles other than protons are in a coronal mass ejection?

In addition to protons, electrons and He nuclei (alpha particles) are the main constituents of the energetic particles released in flares / coronal mass ejections, but trace amounts of other atomic nuclei (carbon, oxygen, iron, etc.) from the Solar atmosphere can also be ejected.

One of your plots seemed to show a several-year recovery time for flares destroying the ozone/atmosphere. If it’s really that long, why?

The ozone depletion due to flares was examined in papers by Segura and colleagues in the journal Astrobiology in 2010 and by Tilley and colleagues (including Segura) in 2019. They showed that both UV and protons, but especially proton fluxes, create a lot of NO and NO2 molecules in an atmosphere like Earth’s (starting as mainly made up of molecular nitrogen and oxygen). Reactions involving NO and NO2 ultimately destroy ozone. As the nitrogen oxides are transported around the atmosphere, they continue to interact chemically with other gasses and trigger ozone depletion. It takes time for all the nitrogen oxides to be removed, so ozone takes a few years to recover. Nitrous oxide generated by humans on Earth depletes ozone in our own atmosphere.

Is the Hubble space telescope obsolete as far as your further intense research?

The Hubble Space Telescope is definitely not obsolete! It’s the only large telescope that will make observations at short-ultraviolet wavelengths, those that don’t get through the Earth’s atmosphere, for the foreseeable future. That’s why we used it to study the flares from Proxima b. Demand for Hubble time is still very great; last year only 18% of proposals submitted got time with Hubble. It also has great capabilities for imaging and taking spectra of disks around stars at visible wavelengths, such as the results on AU Mic that I showed. I’m interested in taking more such images and spectra. It’s also useful for studying exoplanet atmospheres at optical and UV wavelengths that are hard or impossible to access from the ground (or with the next NASA telescope that will launch later this year, the James Webb Space Telescope, which will observe primarily at longer wavelengths). Hubble has detected evidence of exoplanet atmospheres escaping into space, and of atmospheric aerosols (clouds or hazes).


Next month's theme

April is Earth Month, and we think it's a great time to focus our attention on some of our amazing mineralogists who study the literal material that makes up our planet!

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