Centauri Dreams

How is a planet’s composition related to its host star? The European Space Agency’s ARIEL mission (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) is designed to probe the question, examining planetary atmospheres to determine the composition, temperature and chemical processes at work in a large sample of planetary systems.

Transmission spectroscopy is the method, examining spectra as known exoplanets pass in front of, then behind their host stars. Researchers will use light filtering through the atmospheres to unlock the chemical processes within each. ARIEL will survey about 1,000 planetary systems in both visible and infrared wavelengths, probing not just chemistry but the thermal conditions that affect their composition. The mission’s focus is on super-Earths to gas giants, all with temperatures greater than 320 Celsius.

I suspect that principal investigator Giovanna Tinetti (University College London) has been asked about the choice of targets to the point of exhaustion, but one reason for focusing on planets in this size and temperature range is our need to build up a catalog of atmospheres that will inform the entire field, so that when we drill down to small, rocky worlds using future instruments, we’ll have a context in which to place what we see. A high temperature atmosphere is helpful here because it remains in continuous circulation, without the obscuring clouds that make characterization difficult.

Image: Giovanna Tinetti (University College London), principal investigator for ARIEL.

ARIEL will be positioned around L2, the second Sun-Earth Lagrange point, 1.5 million kilometres directly ‘behind’ Earth as viewed from the Sun. A just released ESA report explains the issues the mission will investigate, noting the wide range of planets and stars:

This large and unbiased survey will contribute to answering the first of the four ambitious topics listed in the ESA’s Cosmic Vision: “What are the conditions for planet formation and the emergence of life?”. Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: from rocky Earth-like planets to large gas giants grazing the surface of their host star. There is no known, discernible pattern linking the presence, size, or orbital parameters of a planet to the nature of its parent star. We have little idea whether the chemistry of a planet’s surface and atmosphere is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planet’s birth and evolution.

Remember, too, the statistical nature of the inquiry. The sample population is large, so that we move from the small number of atmospheres currently characterized to hundreds. Our understanding of the early stages of planet and atmosphere formation during the first few million years in an infant system as it emerges from the nebular phase should help us relate the chemistry in exoplanets to their other parameters and to the chemical environment of the star.

Image: An example spectrum Ariel could measure from light passing through an exoplanet’s atmosphere. Credit: ESA/STFC RAL Space/UCL/UK Space Agency/ATG Medialab.

Key components — water vapor, carbon dioxide, methane — of planetary atmospheres will be detected, but also more unusual metallic compounds that define the chemical environment within each system studied. For a smaller number of target planets, the spacecraft will perform a survey of cloud systems and atmospheric changes at a seasonal and daily level. Says ARIEL project scientist Theresa Lueftinger: “Our chemical census of hundreds of solar systems will help us understand each planet in context of the chemical environment and composition of the host star, in turn helping us to better understand our own cosmic neighbourhood.”

ESA has just announced that ARIEL has moved from study to implementation phase, the step before negotiations begin with industrial contractors to build the spacecraft, which is scheduled for launch from Kourou (French Guiana) in 2029. Bids on spacecraft hardware will be requested within months, with the prime contractor chosen by the summer of 2021. 50 institutes from 17 European countries are involved, as is NASA, in the payload module, which will have at its heart a one-meter class cryogenic telescope along with associated science instruments.

Three ESA missions with an exoplanet charter are thus framed within a ten-year window, with ARIEL joined by CHEOPS (CHaracterising ExOPlanet Satellite), launched in December 2019, and PLATO (PLAnetary Transits and Oscillations of stars), to be launched in 2026. The latter emphasizes rocky planets in the habitable zone of Sun-like stars.

To keep up with ARIEL, you may want to follow @ArielTelescope on Twitter. You’ll find background information in the recently published ARIEL Definition Study Report, available here.

We're taking an open approach in data access, even encouraging enthusiasts to help select targets & characterise stars. Much of the data will be available to the science community & public immediately.

(In the meantime check out our Data Challenges! ??)https://t.co/mEEvBrtO5s

— Ariel Space Mission (@ArielTelescope) November 16, 2020

A planetary dynamo may be a key factor in creating the conditions needed for life. And creating that dynamo seems to depend on the radioactive decay of thorium and uranium, generating internal heating and driving plate tectonics. Let’s carry this line of thought further, though, as the authors of a new paper out of UC-Santa Cruz do, and point out that these heavy elements are necessary to create a magnetic field like Earth’s, which protects us from damaging radiation.

In the rocky planets, magnetic fields are generated by convection in a metallic core, which in turn is driven by heat extracted into the mantle (Nimmo 2015; Labrosse 2015; Boujibar et al. 2020). Since mantle radiogenic heat production controls how much heat is extracted from the core, it will also influence the presence or absence of a dynamo. Similarly, heat production will control the mantle temperature and thus the rate of silicate melting and volcanism.

That quote is from the paper, whose lead author is Francis Nimmo, and it points to the key role of radiogenic heat in the mantle. Within the Earth, there are enough radioactive elements to generate a persistent ‘geodynamo,’ in which molten iron and nickel in motion in the planet’s outer core produce electric currents, forming a magnetic field as the Earth rotates. The authors find that Earth “had just enough radiogenic heating to maintain a persistent dynamo.” And that has implications for the atmosphere.

UC-Santa Cruz’ Natalie Batalha is director of the university’s Astrobiology Initiative, whose multidisciplinary work is designed to produce the kind of widespread collaboration found in this paper. Says Batalha:

“It has long been speculated that internal heating drives plate tectonics, which creates carbon cycling and geological activity like volcanism, which produces an atmosphere. And the ability to retain an atmosphere is related to the magnetic field, which is also driven by internal heating.”

To ponder how all these effects come together, we have only to look at Mars, where the lack of a magnetic field can be tied to the gradual loss of an atmosphere that was once thick enough to support water on the surface and, potentially, a living environment. The planet’s low gravity was one factor in the gradual thinning of the atmosphere, but the solar wind of charged particles from the Sun is likewise a problem, for without a protective magnetic field, the atmosphere erodes.

We have an angle here into habitability on planets around other stars, for the amount of radioactive material is going to vary from one stellar system to another. The so-called r-process elements — created by nuclear reactions that are responsible for producing roughly half of the atomic nuclei heavier than iron — owe their origin to supernova events and the mergers of neutron star remnants, events that will be stochastic and result in planets with varying degrees of radiogenic materials. Back to the paper on this point:

From the point of view of long-term radiogenic heating, the most important elements are potassium, thorium, and uranium. Th and U are both r-process elements, most likely produced in neutron star mergers (NSMs) (Kasen et al. 2017; Cowan et al. 2019), with possibly significant contributions also from massive star collapse explosions (Siegel et al. 2019; Macias & Ramirez-Ruiz 2019). Because such events are rare (an occurrence rate of tens/Myr in our Galaxy) and produce large quantities of r-process elements, the resulting concentration of r-process elements should vary considerably. This is especially true of low-metallicity stars, because turbulent mixing of the interstellar medium is inefficient in the early Galaxy (Shen et al. 2015; Naiman et al. 2018). This expectation is in agreement with measurements of r-process europium in low-metallicity stars (e.g., Macias & Ramirez-Ruiz 2018) and with r-process abundances in dwarf galaxies (e.g., Ji et al. 2016).

Lead author Francis Nimmo realized that if planets vary in the amount of radioactive elements, we should then expect geological activity and magnetic fields to vary as well. The UC-Santa Cruz scientists essentially adjusted radiogenic heat production in their models to see how this factor affected planetary habitability. As Nimmo puts it:

“We took a model of the Earth and dialed the amount of internal radiogenic heat production up and down to see what happens. Just by changing this one variable, you sweep through these different scenarios, from geologically dead to Earth-like to extremely volcanic without a dynamo.”

When radiogenic heating is dialed up higher than the Earth’s, no dynamo is produced. With most of the thorium and uranium in the mantle, the excessive heat becomes an insulator, so that the loss of heat from the molten core is slowed. The result: Heat loss from the core cannot generate convection strong enough to produce a magnetic field, with all that implies about the survival of an atmosphere and radiation flux on the surface.

Given all that, the associated volcanic activity produced by greater levels of radiogenic internal heating is almost lost in the shuffle, but it could obviously be a factor in mass extinction events. And while that implies that high levels of radiogenic heating are a key factor in suppressing a habitable environment, the study also shows that too little heat results in no volcanism whatsoever. We wind up with a planet that, like Mars, appears geologically dead.

A new study identifies internal heating from radioactive decay as a critical factor in a planet’s ability to generate a magnetic field and retain an atmosphere. Credit: Illustration by Melissa Weiss for UC Santa Cruz

Image: These illustrations show three versions of a rocky planet with different amounts of internal heating from radioactive elements. The middle planet is Earth-like, with plate tectonics and an internal dynamo generating a magnetic field. The top planet, with more radiogenic heating, has extreme volcanism but no dynamo or magnetic field. The bottom planet, with less radiogenic heating, is geologically “dead,” with no volcanism. Caption credit: UC-Santa Cruz.

So we are working with a single variable, one that can be studied via spectroscopy to measure the abundance of the various elements in stars, the assumption being that planets will have the same composition as the stars they orbit. Here the rare earth element europium comes into play, readily observed in stellar spectra, and created by the same processes that produce thorium and uranium. In this work, europium is the tracer for studying radiogenic abundance.

We already have europium measurements for many nearby stars, which allowed Nimmo and team to set a range of inputs for their models of radiogenic heating, with the Sun’s composition establishing the middle of the range. Europium thus becomes a measurement that can flag radiogenic elements in exoplanetary systems as we look forward to the deployment of the James Webb Space Telescope. The europium measurement may well show us which stellar systems are likely to produce planets with the internal composition life would need to survive. From the paper:

…higher mantle heating rates increase volcanic activity and shut off the dynamo, both likely deleterious for habitability. In contrast, reduced mantle heating rates will at some point stop melt production. A potential consequence is that plate tectonics will then cease (Louren¸co et al. 2018), and with it, the dynamo, as at Venus (Nimmo 2002). It is tempting to speculate that the Earth is habitable in part because it possesses a “Goldilocks” concentration of radiogenic elements: high enough to permit long-lived dynamo activity and plate tectonics, but not so much that extreme volcanism and dynamo shutoff occur. As yet, however, our understanding of the complicated feedbacks involved is too rudimentary to make such a definitive conclusion; more detailed calculations, and better characterization of the radiogenic element abundances in planet-hosting stars, are both required.

The paper is Nimmo et al., “Radiogenic Heating and Its Influence on Rocky Planet Dynamos and Habitability,” Astrophysical Journal Letters Vol. 903, No. 2 (10 November 2020). Abstract / Preprint.

The naming of names is quite a project when it comes to new astronomical objects, and given the sheer numbers — 300 million habitable planets around G- and K-class stars, for example — we might do better to stick with simple identifiers. On the other hand, it’s a bit charming that a new brown dwarf known by its identifier as BDR J1750+3809 has been dubbed ‘Elegast’ by the discovery team. This is the first substellar object found through radio observations.

The name is both appropriate and specific to the discovery space. Elegast appears in a poem in Middle Dutch (12th or 13th Century) called ‘Karel ende Elegast,’ with the character Elegast being a vassal of Charlemagne who seems to be king of the elves (Wikipedia to the rescue, vindicating once again my decision to send them a monthly donation).

The Dutch connection is that the radio work comes out of LOFAR (Low-Frequency Array), which is currently the largest radio telescope operating at the lowest frequencies that we can observe from Earth. Based in the Netherlands, LOFAR is an international effort with field stations in eight countries.

Elegast breaks from the norm in not being discovered from the kind of infrared sky surveys through which most other brown dwarfs have been found. You might recall our discussions of the WISE mission and the possibility, not realized, of finding a brown dwarf closer than the Alpha Centauri stars. But it’s also true that infrared surveys can’t detect objects that are sufficiently cold and faint, whereas we now have radio wavelengths to fill the gap.

Is there still some faint possibility of a closer brown dwarf? T-class dwarfs are defined as being less than 1500 Kelvin, but WISE identified Y-dwarfs with temperatures as cool as 25 °C (298 K), which gets us down to where my thermostat is set as I write this. WISE 1828+2650, I subsequently learned, is so cold that it emits no visible light at all and is more or less indistinguishable from a free-floating planet. Get any colder and infrared surveys won’t find such worlds, which is why the news about Elegast is so encouraging.

Given that the population of so-called ‘rogue’ planets may be vast, it’s wonderful to see a new method becoming available with which to study nearby examples. Brown dwarfs may not be able to create the kind of fusion reactions found in stars like the Sun, but they emit at radio wavelengths just as gas giant planets do. Jupiter’s emissions are well studied, the result of powerful magnetic fields around the planet accelerating charged particles to produce radio waves as well as aurorae. Brown dwarfs are faint at these wavelengths but detectable.

Image: Artist’s impression of Elegast. The blue loops depict the magnetic field lines. Charged particles moving along these lines emit radio waves that LOFAR detected. Some particles eventually reach the poles and generate aurorae similar to the northern lights on Earth. Credit: ASTRON / Danielle Futselaar.

The LOFAR observations had to contend with background galaxies when investigating potential brown dwarfs. The key turned out to be identifying signals that were circularly polarized, which distinguishes planets and brown dwarfs from the background ‘noise.’ What this kind of polarization means is that the electromagnetic field of the radio waves rotates in a characteristic circular pattern. Says LOFAR project scientist Tim Shimwell: “We could not have picked out Elegast in our standard radio images from among the crowd of millions of galaxies, but Elegast immediately stood out when we made circularly polarised images.”

In the case of Elegast, the Gemini-North Telescope had produced archival imagery used to confirm the discovery, and the NASA Infrared Telescope Facility (IRTF) deployed an infrared spectrometer known as SpeX, recently upgraded, to obtain a spectrum of Elegast. The signature of methane, common in brown dwarfs and gas giant planets, was revealed.

Beyond its possibilities at identifying the faintest brown dwarfs and free-floating planets, radio astronomy may also be used to study exoplanet magnetic fields, testing theories on magnetic field strength on objects that have enough similarities to gas giants to make the investigation worthwhile. The team is now making follow-up observations on Elegast to measure its magnetic field even as they use LOFAR to look for more such objects. Harish Vedantham, an astronomer at the Netherlands Institute for Radio Astronomy (ASTRON) is lead author of the study:

“Our ultimate goal is to understand magnetism in exoplanets and how it impacts their ability to host life. Because magnetic phenomena of cold brown dwarfs like Elegast are so similar to what is seen on solar system planets, we expect our work to provide a vital datapoint to test theoretical models that predict the magnetic fields of extrasolar bodies.”

The paper is Vedantham et al., “Direct radio discovery of a cold brown dwarf,” Astrophysical Journal Letters Vol. 903, No. 2 (9 November 2020). Abstract / Preprint.