All posts tagged exoplanets

At our research group meeting on Friday, we discussed an interesting paper from Dr. Tom Barclay and colleagues, which explored how many and what kinds of planets we might find with the TESS mission, launched in April this year.

As Barclay et al. argue, trying to estimate the planet yield from an upcoming survey provides several benefits. For instance, knowing how many planets TESS may find can help astronomers figure out how much time to allot for follow-up observations at large observatories. Also, thinking about TESS discoveries is like staying awake on Christmas eve, anticipating all the presents – it’s just plain exciting.

And make no mistake – TESS will be another game-changer. Kepler focused on figuring out how many of each kind of planet there is in our galaxy, but as one of the trade-offs to facilitate this kind of statistical study, most of the systems found by Kepler are much too far away and dim for us to conduct follow-up studies and learn more about the systems.

TESS takes a different tack, focusing on bright, nearby stars for which additional characterization of the planets will be easier. For instance, some of the planets discovered by TESS will be observed by NASA’s next behemoth, the James Webb Space Telescope, which will reveal the planets’ atmospheres in exquisite detail.

Barclay’s paper lets us shake the presents under the TESS tree, hinting at the goodies inside. By modeling a wide variety of planets in orbit around the 3+ million stars that TESS will see, they try to simulate the kinds of observations the mission will collect and figure out which planets TESS can find easily and which ones it will struggle with.

For example, they find TESS may discover nearly 300 planets with radii smaller than twice the Earth’s.  Among these potentially Earth-like planets, roughly ten will orbit in the temperate zone, making them  possible oases for extraterrestrial life.

After reading these results about potentially habitable planets, I was also excited about their prediction that TESS may find 12,000+ giant (i.e. Jupiter-sized) planets. Barclay et al. caution that these objects will be especially hard to distinguish from astrophysical false positives. But these planets may also reveal some of the most interesting astrophysical phenomena, so if there are any clever tricks to extricate these planets, the effort might prove worthwhile.

A pine grove on Puget Sound’s campus.

Just recently returned from the meeting of the Northwestern Section of the American Physical Society, which took place on the lovely campus of the University of Puget Sound.

It was a cozy meeting of physicists and student physicists from throughout the northwest, and there was a variety of talks and posters on topics from gravitational waves to DNA computers to diversity in science.

One of the talks that really stuck out in my mind was the banquet presentation from Puget Sound’s Prof. James Evans about the antikythera mechanism, a mysterious barnacle-encrusted gearwork recovered from an ancient Grecian shipwreck.

Constructed by an unknown artisan in about 200 BC (according to Evans), the machine could track the date, follow lunar phases, predict solar eclipses, and even maybe show planetary motions  — all with the turn of a single crank.

For my presentation, I gave a brief overview of exoplanet astronomy, with a focus on how these discoveries have begun to hone our ideas about alien life and extraterrestrial civilizations. I’ve posted by presentation below.

With the very first discovery of an exoplanet around a Sun-like star (51 Peg b), astronomers were introduced to hot Jupiters. These totally unexpected planets resemble Jupiter in mass, composition, and size, but they have orbits that nearly skim the surfaces of their host stars. Some of them are even losing their atmospheres under the apocalyptic glare of their host stars.

How their lives began remains a mystery, but we have a pretty good idea of how their lives will end – they will be engulfed or torn apart by their host stars. That’s because hot Jupiters are big and close enough that they can actually raise a tidal bulge on the stars (and we can actually see the bulge in a handful of cases).

This tidal interaction can cause the planets to spiral downward toward the stars, and at the same time, it causes the spins to spin faster until the planet is destroyed by the star. The same tidal effect, just in reverse, is driving the Moon away from the Earth, while slowing down the Earth’s spin. But here’s the key: we don’t know how quickly the planets are spiraling in.

Tidal decay of planetary orbital period over billions of years (Gyrs). From Penev et al. (2018 – https://arxiv.org/abs/1802.05269).

Enter Prof. Kaloyan Penev of UT Dallas Physics Dept. On Valentine’s Day last week, he and his colleagues published an academic love note exploring planetary tidal decay. To do this, they modeled the evolution of planetary orbits and stellar spins under the influence of tides. The tracks in the figure at left show how a planet’s orbital period (or distance from its star) might shrink over billions of years, thanks to tides. The clump of spaghetti noodles in the figure shows that evolution for a range of assumptions about the rate of decay.

By comparing the stellar spin rate and planetary orbit predicted by their model to those we actually observe for each system, Penev and colleagues showed that the tidal decay rates might actually slow down as the planets approach their stars. So perhaps instead of an reckless death dive into the star over a few million years, the planets make like Zeno’s tortoise and tiptoe closer and closer without plunging in.

Upcoming surveys such as the TESS mission and the Large Synoptic Survey Telescope may soon allow us to test whether planets do or do not plunge into their stars. Theoretically, we expect stars that eat their planetary children dramatically brighten up by a factor of 10,000 over a few days – faster than a supernova brightens but nowhere near as bright. These surveys might able to see stars engaged in this act of cosmic infanticide.

Nominal trajectory of ‘Oumuamua. From https://en.wikipedia.org/wiki/File:C2017U1.gif.

A month ago, astronomers found, for the first time, an asteroid that definitely originated from outside our solar system.

The object, 1I/ʻOumuamua, came screaming into our solar system at 60,000 mph, took a sharp turn around the Sun, and passed within 10 million miles of Earth on Oct 18 before beginning its long journey out of our solar system and back into interstellar space.

Given its highly elongated and inclined orbit, ʻOumuamua was initially classified as a comet, but follow-up observations showed no sign of a coma, and so it was re-classified as an asteroid. Its discovery has prompted a flurry of short but exciting astronomical studies, and in our research group meeting this week, we discussed two: Ye et al. (2017) and Laughlin & Batygin (2017).

In their study, Ye and colleagues describe their observations of ʻOumuamua’s brightness and color. Their color observations indicated that ʻOumuamua is slightly but not very red, unlike many icy bodies in our Kuiper Belt. This result suggests it either formed close to its original central star (and never had much ice) or spent time near enough to its original parent star to have baked off any ice.

They also estimated that ʻOumuamua passed very near Earth’s orbit, close enough that, if any material were ejected from its surface, it may produce a meteor shower in a few hundred years.

In their study, Laughlin and Batygin took a more theoretical tact and explored possible implications of ʻOumuamua’s for the existence of planets like the putative Planet Nine.

ʻOumuamua almost definitely originated in a distant solar system and was ejected by a gravitational interaction with a planet in that system, and Laughlin and Batygin point out that most of the known exoplanet population would probably not be very good at ejecting objects like ʻOumuamua: these planets are so small and/or close to their host stars that they cannot easily liberate asteroids like ʻOumuamua from the host stars’ gravitational clutches.

But, Laughlin and Batygin suggest, if there is a sizable population of largish (several Earth masses) planets several times farther from their host stars than Earth is from the Sun, then gravitational ejections of asteroids might occur frequently enough to explain objects like ʻOumuamua.

Granted, they’re dealing with a sample size of one, but several all-sky surveys, like LSST and TESS, will arrive on the scene any day now. And we may very soon find other interstellar interlopers like ʻOumuamua. The galaxy is probably full of them.

Figure 1 (left) and 2 (right) from Anglada et al. (2017). The right figure shows a zoomed-in version of the left figure. The rainbow blob at the center of the left figure is Proxima Centauri’s debris disk, and the white ellipse shows the possible outer disk. The greenish blob just to the left of center in the right figure is the mysterious source, possibly a ringed planet.

In case you didn’t hear, late last year, astronomers confirmed a planet around our nearest stellar neighbor, Proxima Centauri, a red-dwarf star just four light years from Earth. The planet is probably about 30% more massive than Earth, probably making its composition Earth-like, and it’s in the habitable zone of its star, at a distance of about 0.05 astronomical units (AU) – all of which make it an exciting prospect for follow-up studies.

And just last week, Guillem Anglada and colleagues announced the further discovery of a debris disk around the star. The left figure up top shows the image, in radio wavelengths, of emission from the disk – the disk appears as the rainbow blob near the center, and the location of the host star Proxima is marked with a black cross.

The disk’s appears to orbit between 1 and 4 AU from its host star, which would put it between the Earth and Jupiter if it orbited in our solar system. However, since the red-dwarf star is so much smaller and cooler than our Sun, those orbital distances correspond to temperatures of only a few tens of degrees, making Proxima’s disk more akin to our Kuiper belt than our main asteroid belt.

The radio light we see from the disk is mostly due to thermal emission from dust. Using the above temperature estimate (and some other reasonable assumptions), Anglada and colleagues estimate (with large uncertainties) Proxima’s disk has about one thirtieth the mass of Ceres in dust and a lunar mass in larger bodies – almost as much mass as our Kuiper belt. There’s also marginal evidence in the data for a larger and cooler disk as well, perhaps 30 times farther from the star than the inner disk, and for something perhaps even more interesting.

In the right figure above, see the greenish blob just below and to left of the rainbow blob? That (admittedly weak) signal could be emission from a ring system orbiting a roughly Saturn-mass planet about 1.6 AU distant from the star. The authors point out that there’s a small but non-zero chance that it’s actually just a background galaxy that photobombed their observations, a possibility that can be easily tested by looking at Proxima again in a few months. But if it turns out to be a ringed planet, it would be the first exo-ring system directly imaged (other systems show possible signs of rings).

That would make Proxima an even more unusual planetary system since small stars tend to have small planets, and I’m only familiar with one other red dwarf star that hosts a big planet – NGTS-1 b, a red-dwarf hosting a hot Jupiter. But if there’s one thing that exoplanet astronomy has taught us in the last few decades, it’s to expect the unexpected.

The diagram below shows the structure of the Proxima Centauri system suggested by Anglada and colleagues.

Figure 4 from Anglada et al. (2017), showing the suggested structure of the Proxima Centauri planet-disk system.

This is the sharpest image ever taken by ALMA — sharper than is routinely achieved in visible light with the NASA/ESA Hubble Space Telescope. It shows the protoplanetary disc surrounding the young star HL Tauri.

Exoplanets are being discovered from near and far, and one way to learn more about how these planets form is to study the disk of gas and dust from which they form.

Join the Boise State Physics Department on Friday, Oct 6 at 7:30p in the Multi-Purpose Classroom Building, room 101 to hear Prof. Hannah Jang-Condell of the University of Wyoming discuss her cutting-edge research on these protoplanetary disks and how the telescopes at University of Wyoming are being used to better understand and characterize exoplanets.

At 8:30p after the presentation, we will stargaze on the roof of the Brady Street Parking Garage, weather permitting.

The event is free and open to the public.

Figure from Mocquet et al. (2014) show how a gaseous planet might evolve into a dense, rocky core.

Another blast from the past, Mocquet et al. (2014) was the topic of our journal club this week, a paper that seeks to answer the question “What would Jupiter look like if you took away its atmosphere?”.

Given the enormous number of gas-rich exoplanets very close to their host stars discovered in recent years, many astronomers (including myself) have wondered whether such planets could have their atmospheres completely removed.

We certainly see some very hot planets where intense sunlight is blasting away their atmospheres, and in other cases, the star’s gravity can rip off the atmosphere. And so it’s not crazy to think some gaseous planets might completely lose their atmospheres.

Would anything be left over? Astronomers think that gas giants like Jupiter are like big cherries, with a squishy outer layer of gas wrapped around a dense pit of rock. Indeed, the Juno mission currently in orbit around Jupiter is designed to measure the size of Jupiter’s core by measuring its gravitational field very precisely.

And so the cores of gas giants are under enormous pressure – for instance, the core of Jupiter is being squeezed by 45,000 times the pressure at the bottom of the Mariana Trench on Earth.

In their study, Mocquet and colleagues explore what happens to a rocky core under such large pressures. Not surprisingly, they find that such a core would have an enormous density, perhaps three times larger than the Earth’s.

But what is surprisingly is that their results suggest the core might retain a very large density even if you removed the overlying atmosphere. It’s as if you squeezed down a nerf ball and then let it go – instead of springing back immediately, the nerf ball would take a few billion years to decompress. This means that we might be able to identify the cores of former gas giants by looking for planets roughly the size of Earth but with anomalously high densities.

And in fact, such planets have been found – the planet Kepler-57 b has a mass more than 100 times Earth’s but squeezed into a volume only ten times larger, giving a density of almost 44 grams per cubic centimeter – twice the density of the densest element on Earth, osmium.

So in their search for fossils, gas giant paleontologists should keep in mind that the bones of extinct gas giants may have distinctively large densities, almost as dense as adamantium.

Artist’s conception of a protoplanetary disk from which planets form.

During today’s research group meeting, we discussed a paper from a few years ago from Lars Buchhave and colleagues that investigated the relationship between the composition of a planet-hosting star and the properties of its planets.

The discoveries of thousands of exoplanetary systems in the last few decades has revealed the bewildering variety of planets formed in our galaxy, and the richness of this planetary zoo probably reflects the wide range of conditions in which these planets formed.

Going back to the philosopher Kant, planets have been thought to form in disks of gas and dust leftover after their host star forms, and we now have a plethora of observational and theoretical evidence supporting this idea.

This idea means that the star and planets form mostly from the same source of material. However, while stars form directly out of the disk, the formation process for planets is a little pickier about what goes into the planets.

For example, the Sun is made almost entirely out of hydrogen and helium, elements that constitute most of the baryonic matter in the universe, while the Earth is made mostly of rocky elements, which are pretty rare in the universe. The gas giant Jupiter is kind of a mix – it’s mostly hydrogen and helium like the Sun, but it has more of the heavier elements than the Sun, all of which astronomers refer to as metals.

In their paper, Bucchave and colleagues report estimates of the ‘metallicities‘ or the amount of metals in lots of planet-hosting stars and try to figure if the type of planets around a star depends somehow on stellar metallicity.

Figure 1 from Bucchave et al. (2014) shows the metallicities of stars vs. the radii (in Earth radii) of their planets. The horizontal red lines show the average metallicity for stars in that group.

Interestingly, the metallicities suggests there are three kinds of planetary systems – shown as dark blue, light blue, and yellow in the figure above. Big gaseous planets like Jupiter, with radii many times Earth’s, seem to form preferentially around stars with lots of metals, while small planets like the Earth aren’t as picky – they’ll form around stars with any metallicity. And planets with radii in between, about 2 to 4 times the Earth’s radius, they’re like Goldilocks and prefer stars with a little more metals but not too much.

What does all this mean? Astronomers think the protoplanetary disk (and therefore the star) might be required to have lots of planet-forming materials (that is, metals) in order to make big planets like Jupiter. On the other hand, forming small planets like the Earth apparently doesn’t take much because even stars with a tenth the Sun’s metals host them. Which all sort of makes sense.

But these results don’t answer everything. Why, for example, aren’t the stars with really big metallicities (the blue dots near the top left of the figure) always able to form big, Jupiter-like planets? This cluster of three blue dots are all members of the KOI-3083 planet system, whose star is Sun-sized but has almost three times more metals, but all the planets are smaller than Earth.

Could there be big planets in that system we haven’t found yet? Or maybe the planet formation process involves so much randomness (stochasticity) that a big metallicity only steers the system in the direction of big planets; it doesn’t force them in that direction. Like gently shepherding a toddler through a toy store – more often than not, you’ll end up with toys in your cart.