Centauri Dreams

An iron and nickel-rich planetesimal is apparently all that survives of a planet following the death of its star, SDSS J122859.93+104032.9. We are talking about an object in an orbit around a white dwarf so tight that it completes a revolution every two hours. Significantly, spectroscopic methods were used to make the identification, the first time a solid body has been found around a white dwarf with spectroscopy. Variations in emitted light were used to identify the gases generated by the planetesimal, with data from the Gran Telescopio Canarias in La Palma.

Lead author Christopher Manser (University of Warwick) notes the advantages of the method the team developed to study a white dwarf 400 light years away:

“Our discovery is only the second solid planetesimal found in a tight orbit around a white dwarf, with the previous one found because debris passing in front of the star blocked some of its light — that is the “transit method” widely used to discover exoplanets around Sun-like stars. To find such transits, the geometry under which we view them has to be very finely tuned, which means that each system observed for several hours mostly leads to nothing. The spectroscopic method we developed in this research can detect close-in planetesimals without the need for a specific alignment.”

Image: A planetary fragment orbits the star SDSS J122859.93+104032.9, leaving a tail of gas in its wake. Credit & copyright: University of Warwick/Mark Garlick.

This is an extreme environment, the white dwarf in question being surrounded by a debris disk through which the object passes in its orbit. The star itself is about 70 percent of the mass of the Sun and, like all white dwarfs, this one — roughly the size of Earth — is quite dense, a survivor of the star’s red giant phase. An object moving this close to the white dwarf will be under extreme gravitational stress; the gravity of SDSS J122859.93+104032.9 is fully 100,000 times that of the Earth. The fact that the team could identify a planetesimal deep within the gravitational well indicates it must be an object of great density, probably made up of iron and nickel.

On where the object came from, the paper offers this intriguing possibility:

This object may be the differentiated iron core of a larger body that has been stripped of its crust and mantle by the tidal forces of the white dwarf. The outer layers of such a body would be less dense and would disrupt at greater semimajor axes and longer periods than those required for core disruption. This disrupted material would then form a disc of dusty debris around SDSS J1228+1040, leaving a stripped corelike planetesimal orbiting within it.

Manser’s colleague and co-author Boris Gaensicke adds if the assumption that we are dealing with a planetary core is correct, then the original body would have been at least hundreds of kilometers in diameter, because it is only at this size that planets begin to differentiate, with heavier elements sinking to form a metal core. It could, of course, have been much larger.

Thus the survival of a planetesimal here, actually orbiting within the original radius of its star, suggests a large object ultimately shredded by gravitational forces. We are glimpsing what our own Solar System may resemble in 5 to 6 billion years, when it will be a white dwarf orbited by the outer planets along with asteroids and comets. Our star’s expansion into a red giant will savage the inner system, perhaps leaving debris like what we see around SDSS J122859.93+104032.9. Bear in mind, too, that the vast majority of the stars known to host planets will end their lives as white dwarfs, so we are looking at a common destiny.

The debris disk of the white dwarf is rich in magnesium, iron, silicon and oxygen, and it is within that disk that the scientists found gas streaming from the evidently solid body. The object appears to be about a kilometer in size but could be as large as a few hundred kilometers in diameter. Whether it is the source of the gas or simply the cause of the gaseous ‘tail’ as it collides with debris in the disk is not yet known. Learning more will involve studying other debris disks similar to SDSS J122859.93+104032.9 (eight gaseous white dwarf debris discs are currently known), where the spectroscopic method will perhaps find other instances of planetesimals orbiting near or within the parent star’s debris disk.

The paper is Manser et al., “A Planetesimal Orbiting Within the Debris Disc Around a White Dwarf Star,” Science April 4 2019 (abstract).

A method for enhanced exoplanet investigation takes center stage today as we look at the GRAVITY instrument, a near-infrared tool aided by adaptive optics that brings new precision to exoplanet imaging. In operation at the European Southern Observatory’s Very Large Telescope Interferometer (VLTI) at Paranal Observatory in Chile, GRAVITY works with the combined light of multiple telescopes to produce what would otherwise take a single telescope with a mirror diameter of 100 meters to equal. The early demonstrator target is exoplanet HR 8799e.

The method at work is interferometry, and here we are applying it to a ‘super Jupiter,’ more massive and much younger (at 30 million years) than any planet in our Solar System. The GRAVITY observations of this target mark the first time that optical interferometry has been used to study an exoplanet at this level of precision, producing a highly detailed spectrum. The planet is part of a 5-planet system some 130 light years away, all 5 of the planets being gas giants between 5 and 10 times the mass of Jupiter.

Image: This wide-field image shows the surroundings of the young star HR 8799 in the constellation of Pegasus. This picture was created from material forming part of the Digitized Sky Survey 2. Credit: ESO/Digitized Sky Survey 2. Acknowledgement: Davide de Martin.

The high resolution images that resulted from this work show what we can expect from optical interferometry going forward. We now know the distance between HR8799e and its star with 10 times the accuracy of previous estimates, which in turn helps to refine the planet’s orbit, one that appears to be slightly inclined compared to the orbital plane of its four companions.

That high-grade spectrum has spoken volumes about the composition of the planet’s atmosphere, says team leader Sylvestre Lacour (Observatoire de Paris and the Max Planck Institute for Extraterrestrial Physics):

“Our analysis showed that HR8799e has an atmosphere containing far more carbon monoxide than methane — something not expected from equilibrium chemistry. We can best explain this surprising result with high vertical winds within the atmosphere preventing the carbon monoxide from reacting with hydrogen to form methane.”

Also present here are clouds of iron and silicate dust, all suggestive of violent storms, as convection causes the dust to rise and then descend into the interior. We’re seeing a giant planet in its turbulent infancy. And what an impressive demonstration of interferometry’s ability to separate star and, in this case, a very close planet, with a result that the European Southern Observatory considers much cleaner than what could be achieved with a coronagraph that would mask out the light of the star.

Image: Exoplanet HR 8799e has been analyzed spectroscopically separate from the parent star HR 8799 using the new technique (artistic impression). Credit: © ESO/Luis Calçada.

We’ll be using analyses of planetary atmospheres to look for biosignatures one day on cooler and more clement worlds. In the interim, astronomers plan to continue the investigation of the HR8799e system, allowing so complete an analysis of the planet’s orbit that it will be used to reveal the mutual gravitational interactions of the giant planets as well as the influence of the central star. That, in turn, takes us to accurate estimates of the planetary masses.

And these excerpts from the paper are notable. In the first, ‘Kmag’ refers to ‘K magnitude,’ the magnitude of the star about which the extrasolar planet orbits as viewed through a specific filter at near-infrared wavelengths, in this case between 2.0 and 2.4 ?m. GRAVITY operates within the K band:

The interferometric technique brings unique possibilities to characterize exoplanets. With the technique described here, any planet with K ? 19, ?Kmag ? 11, and separation ? 100 mas is, in theory, observable with GRAVITY. The numbers are still to be refined, but it would mean that GRAVITY could observe most of the known imaged planets, and maybe in the near future planets detected by radial velocity [italics mine].

And this:

…the idea that an interferometer can resolve the surface of exoplanets, giving radius and resolving clouds patchiness, is now becoming more plausible. However, it would require an interferometer with baselines on the order of 10 km. This could be a goal for ESO after ELT construction.

The paper is Lacour et al., “First direct detection of an exoplanet by optical interferometry,” in Astronomy and Astrophysics, Vol. 623 (March 2019), L11 (abstract / preprint).

Picking up on TESS (Transiting Exoplanet Survey Satellite), one of whose discoveries we examined yesterday, comes news of a document called the “TESS Habitable Zone Star Catalog.” The work of Cornell astronomers in collaboration with colleagues at Lehigh and Vanderbilt, the paper has just been published in Astrophysical Journal Letters (citation below), where we find 1,822 stars where TESS may find rocky terrestrial planets.

The listed 1,822 are nearby stars, bright, cool dwarfs, with temperatures roughly between 2,700 and 5,000 Kelvin, with a TESS magnitude brighter than 12 and reliable data from the Gaia Data Release 2 as to distance. Here TESS can detect 2 transits of planets that receive stellar irradiation similar to Earth’s, during the 2-year prime mission. 408 of these stars would allow TESS to detect transiting planets down to Earth size during one transit. The catalog is fine-tuned to the TESS instrumentation and mission parameters, the stars selected because they offer sufficient observing time to be able make these detections.

From the paper:

What distinguishes this catalog from previous work like HabCat (Turnbull & Tartar 2003), DASSC (Kaltenegger et al 2010) and CELESTA (Chandler, McDonald, & Kane 2016) is that the stars included here are specifically selected to have sufficient observation time by TESS for transit detection out to the Earth-equivalent orbital distance. We also use Gaia DR2 data, which allows us to exclude giant stars from the star sample and provides reliable distances for our full star sample. All the stars have been included in the TESS exoplanet Candidate Target List, ensuring that they will also have 2-minute cadence observation (provided they do not fall in TESS camera pixel gaps), providing a specific catalog for the TESS mission of stars where planets in the Habitable Zone can be detected by TESS. This data will be available to the community in the ongoing public TESS data releases.

In case you’re wondering, 137 stars in the catalog are within the continuous viewing zone of the James Webb Space Telescope, which will be able to observe them to characterize planetary atmospheres and search for biosignatures. Many more will be followed up after any TESS planet identifications by ground-based extremely large telescopes currently under construction.

Image: The TESS search space compared to that of the Kepler Mission. Credit: Zach Berta-Thompson.

The idea, then, is to help us shape our target lists for TESS by pointing to the most likely places of discovery. Meanwhile, Elisa Quintana (NASA GSFC) has been thinking about planets we can’t yet detect but which may indeed be present, using Kepler data as massaged by a mathematical model that has implications for TESS and future mission datasets. The difference is that in Quintana’s case, these are systems where we already know planets exist. The question: What other planets might yet be found in the same systems?

After all, we have to wonder what our methods may have missed. The Kepler mission has led us to believe that most stars in the galaxy have planetary companions, but around even relatively close stars, we may be seeing a subset of what’s actually there. Using the transit method, which Kepler employed to such brilliant effect, we only see the planets that move across the face of the star as seen from Earth. There could be others in the same system that do not.

Think about how rare a transit of Venus is. Even from our vantage so close to the planet, we see Venus cross the Sun only in pairs of transits eight years apart, separated by gaps of over a century. Indeed, the last transit of Venus of the 21st Century has already taken place (5,6 June, 2012); we have to wait until December of 2117 for the next. The orbit of Venus is responsible for the rarity of the phenomenon; it’s inclined by 3.4° relative to the Earth’s orbit. Exoplanetary systems are presumably not immune to such variation.

Quintana has been working as mentor with an 18 year-old high school student named Ana Humphrey, who developed the model to predict possible planets in such systems. Out of Humphrey’s work, which has garnered a a $250,000 prize in the Regeneron Science Talent Search, we learn that there may be as many as 560 ‘hidden’ planets in exoplanet systems identified by Kepler. Says Humphrey:

“I was completely fascinated by this idea of finding new planets using mass, based on data that we already had. I think it just shows that even if your data collection is complete, there’s always new questions that can be asked and can be answered.”

Image: Ana Humphrey won a $250,000 prize for calculating the potential for finding more planets outside our solar system. Credit: NASA GSFC.

Indeed, as Quintana points out, systems like Kepler-186 show a large gap that exists between the four planets close in to the star and the outer planet. Another world the size of Earth could be there on an orbit inclined enough that we would not see it. Extend this over the range of multi-planet systems found thus far and there is ample room for additional discovery. Humphrey’s model manipulates the possible space between the hypothetical planet and its neighbors, to see what worlds of varying mass could be present without disrupting their orbits.

This could come in handy for TESS, which as we saw yesterday, is already producing planetary discoveries like TOI-197. Applying the new model to the exoplanet database being assembled by TESS would allow both it and future missions to predict systems in which hidden planets might be found. Such systems might then be studied both by transits and other methods.

In examining such questions, Quintana and Humphrey are simply extending a time-honored method of planetary discovery, one that led Johann Gottfried Galle, working with calculations from Urbain Le Verrier, to discover Neptune in 1846 (and yes, Neptune was observed before this but was not known to be a planet). The mathematical calculations that produced Neptune as planet captured the imagination of François Arago, who said that Le Verrier had discovered a planet “with the point of his pen.” Thus does one world grow out of another — it was data on Uranus and the irregularities of its orbit that led to our learning the true nature of Neptune.

Remarkably, Triton was discovered a mere 17 days after the discovery of Neptune, another case of data cascade. Applying the same concept to exoplanets has been a natural progression. We can actually see only a few such worlds through direct imaging. Fine-tuning our models to fit the methods and instruments at hand maximizes the opportunity to enlarge our catalog.

The paper is Kaltenegger et al., “TESS Habitable Zone Star Catalog,” Astrophysical Journal Letters Vol. 874, No. 1 (26 March 2019). Abstract / preprint.

It’s good to see TESS, the Transiting Exoplanet Survey Satellite, producing early results. We’re coming up on the one year anniversary of its launch last April 18, with the spacecraft’s four cameras doing month-long stares at 26 vertical strips of sky, beginning with the southern hemisphere. Two years of such scanning will produce coverage of 85 percent of the sky.

The focus on bright, nearby stars is a shift from the Kepler strategy. While both missions have dealt with planetary transits across the face of their star as seen from the spacecraft, TESS is going to be producing plenty of data for follow-ups, planets close enough that we can consider studying their atmospheres with future missions beginning with the James Webb Space Telescope. Kepler’s long stare was of distant stars in a specific region, the idea being to gain a statistical understanding of the prevalence of planets. TESS gets us closer to home.

Now we have TOI-197 (TOI stands for ‘TESS Object of Interest’), a planet close to the size of Saturn in a tight orbit of its star (about 14 days). Asteroseismology comes into play here, with astronomers from the TESS Asteroseismic Science Consortium (TASC) using stellar oscillations to make a call on the star’s age, about 5 billion years. The star turns out to be slightly larger than the Sun, a late subgiant / early red giant, while the candidate planet shows a radius 9 times that of Earth. It masses 60 Earths, and its density is 1/13th that of our own planet.

Image: A “hot Saturn” passes in front of its host star in this illustration. Astronomers who study stars used “starquakes” to characterize the star, which provided critical information about the planet. Credit: Gabriel Perez Diaz, Instituto de Astrofísica de Canarias.

Asteroseismologists examine seismic waves in stars (think of them as ‘starquakes’) that show up as changes in brightness, and offer useful clues about about radius, mass and age. TOI-197 turns out to be the first TESS planet for which the oscillations of the host star can be measured. It’s interesting to see, then, that in addition to the paper on TOI-197, we also have a new paper that will prove useful for TESS characterizations going forward. It’s a target list, prepared by TASC, that identifies some 25,000 stars that are both Sun-like and oscillating.

So the asteroseismology work with TESS data gets underway. Steve Kawaler (Iowa State University) notes the scope of the work:

“The thing that’s exciting is that TESS is the only game in town for awhile and the data are so good that we’re planning to try to do science we hadn’t thought about. Maybe we can also look at the very faint stars – the white dwarfs – that are my first love and represent the future of our sun and solar system.”

An interesting thought given shape by the TOI-197 findings that reveal TESS’ potential for asteroseismology. From the discovery paper:

TOI-197 provides a first glimpse at the strong potential of TESS to characterize exoplanets using asteroseismology. TOI-197.01 has one the most precisely characterized densities of known Saturn-sized planets to date, with an uncertainty of ? 15%. Thanks to asteroseismology the planet density uncertainty is dominated by measurements of the transit depth and the radial velocity amplitude, and thus can be expected to further decrease with continued transit observations and radial velocity follow-up, which is readily performed given the brightness (V=8) of the star. Ensemble studies of such precisely characterized planets orbiting oscillating subgiants can be expected to yield significant new insights on the effects of stellar evolution on exoplanets, complementing current intensive efforts to characterize planets orbiting dwarfs.

We also have in TOI-197 an addition to the list of close-in transiting worlds around evolved stars; i.e., stars that have begun their transition to red giant status. Worlds like this undergo what the paper calls ‘radius reinflation’ as the host star evolves up the red giant branch. According to the paper, TESS is expected to “detect oscillations in thousands of main-sequence, subgiant and early red-giant stars… and simulations predict that at least 100 of these will host transiting or non-transiting exoplanets.”

The papers are Schofield et al., “The Asteroseismic Target List for Solar-like Oscillators Observed in 2 minute Cadence with the Transiting Exoplanet Survey Satellite,” Astrophysical Journal Supplement Series Vol. 241, No. 1 (14 March 2019). Abstract; and Huber et al., “A Hot Saturn Orbiting an Oscillating Late Subgiant Discovered by TESS,” accepted at the Astronomical Journal (preprint).