Category Archives: Historical

Revelation

Pluto as seen by New Horizons on 13 July 2015

Pluto as seen by New Horizons on 13 July 2015

Tomorrow is a rather big day for the exploration of the Solar System. A world that has long represented our ignorance about the outer solar system will come into splendid view, as revealed by the New Horizons spacecraft. Pluto, a dwarf planet discovered in 1930, has always been that question-mark at the end of the book about the Solar System.

I tend not to cover solar system exploration on this blog because it’s dedicated to extrasolar planet science, but I think there are some interesting parallels one can make between Pluto and extrasolar planets. Until recently, Pluto has just been an unresolved dot in the sky. Indirect methods had allowed for the mapping of crude surface features — we came to learn that Pluto has significant albedo variation across its surface, with patches of bright and dark, but other than that, we knew nothing about their composition or even what caused them. To a large extent, we still don’t. We now know (as of the past couple days) that Pluto’s north pole is covered in an ice cap dominated by nitrogen and methane, and that the dark regions are comparably methane-poor, but we don’t really understand a lot of what’s going on yet. But the advance in our knowledge of Pluto over the past month has been truly revolutionary.

In the not-too-distant future, indirect methods will begin to yield crude albedo maps of extrasolar planets. These maps may have a similar quality to those acquired for Pluto. It will be worth remembering, however, that there’s so much more about those planets that we won’t be able to see simply because we lack the ability to send a probe of some sort to those extreme distances.

New Horizons Map Comparison

New Horizons Map Comparison

The image above shows a comparison between our “best map” of Pluto before New Horizons, and what is currently our limit of knowledge (with full credit to Bjorn Johnson). Quite a difference a spacecraft visit makes! The top map is an average of five separate maps acquired by HST and ground photometry, so an important caveat here is that not only is the wavelength coverage different than New Horizons’ LORRI camera made to use the bottom map, but the mapping technique is different, too, and prone to different biases. That all being said, there is a decent amount of similarity between the two.

We may live to see an exoplanet’s surface resolved to an extent similar to Pluto’s before New Horizons, however it will be several generations before we are able to map an exoplanet with the same level of precision as the bottom map in the image above.

2014 Review

Comets

Comets orbiting β Pic (Credit: ESO)

Results from Kepler data continued to stream in. We were graced with the discovery of a transiting Uranus-sized planet in a 704-day orbit. This is the first time a transiting planet has been discovered beyond its system’s ice line, and this may even be where the planet formed, instead of the typical case for transiting planets where we see them were they are today because of extensive inward migration in their past. Kepler’s 10th transiting circumbinary planet was reported, Kepler’s second multi-planet system orbiting a sdB star was announced and Kepler’s second disintegrating short-period planet was identified.
Kepler short-cadence data may have allowed for the detection of a non-spherical (oblate, like Saturn) exoplanet for the first time, by looking at photometry for Kepler-39 b. While the planet itself is unremarkable, a transiting sub-Neptune around HIP 116454 marks an exciting development: The first planet discovered by Kepler during it’s new “K2” mission.

The dramatic turn of events in the investigation of the planetary system at GJ 581 seems to have finally come to a conclusion. In 2007 a media frenzy accompanied the discovery of two new planets found to accompany the known 5-day Neptune. The innermost of the two new planets was a 5 Earth-mass planet in a 12-day orbit, and a 7 Earth-mass planet in a 80-day orbit. The 12-day planet was hailed as the first habitable planet candidate, despite getting more stellar insolation than Venus. In 2009, an Earth-mass planet at 3-days was found, and the new dataset brought the orbit of the 80-day planet closer to the star, down to ~60 days. Since most people had come to their senses regarding the habitability of the 12-day planet, the 60-day planet became the flag-bearer for habitability in the GJ 581 system. Everything changed in late 2010 when a 3 Earth-mass planet in the habitable zone was announced by the HARPS team. This was truly the most habitable exoplanet candidate known to date… if it were real. Multiple studies drawn out over the following several years debated back and forth how many planets exist around GJ 581. Three? Four? Five? Six? It depended on how you merged your datasets, how you handled noise in your data, and so on. This year, the issue seems to have been resolved by carefully examining the H-alpha lines of the star’s spectrum and finding variation in them that had affected the stellar radial velocity measurements. When corrected for, suddenly all of the habitable zone planets vanished. As of the end of 2014, there are no habitable planet candidates orbiting the star that only a few years ago was unanymously acknowledged to be the most promising extrasolar planetary system for habitability. Interestingly, a similar story may be unfolding at GJ 667, where some planet signals have turned up missing in other studies of the RV data. The morale of this story? Finding low-mass planets is hard.

HARPS continued to give us new planets, but the pace seemed somewhat slow. Some interesting results are the presence of planets in the cluster M67 (link). Planets were found orbiting the stellar companion to XO-2, as well, making the system an example of a binary system with planets orbiting both components. Interestingly, both host at least two planets, and one of each of them are a hot Jupiter. The hot Jupiter around the other star doesn’t transit, suggesting some misalignment between the two planetary systems. Other HARPS results included the identification of two families of comets around β Pictoris, with the identification of hundreds of individual comets through via transmission spectroscopy through their tails.

The news media nearly peed their pants with excitement over the discovery of a planet that, most optimistically, isn’t habitable now, nor has it ever been, in the habitable zone of the very nearby Kapteyn’s Star. Similar unwarranted attention was given to a mini-Neptune discovered in the habitable zone of GJ 832. While the excitement that planets like these get is unwarranted, it is at least gratifying to see the general public so interested in planets that vaguely resemble something habitable-ish. I just hope that when we do find more planets that are more like Earth, the interest hasn’t faded away already. HIRES also found a super-Earth orbiting the very nearby star GJ 15.

We saw first results from the Automated Planet Finder, a radial velocity based system to look for planets in the solar neighbourhood. The project gave us a nice four-planet system orbiting HD 141399 and aided in the discovery of a Neptune-mass planet at GJ 687. We also got to see first results from ESO’s SPHERE instrument.

A number of naked-eye stars were found to have planets, as well, such as planets around β Cnc, μ Leo, β UMi and one around σ Per.

On the direct imaging, we got a new planet imaged orbiting GU Psc with a mass ratio of ~30 and a separation of ~2000 AU… which seems more like a low-mass binary star than a true planetary system. Also, while not planets, ALMA observations of HL Tau have revealed increible detail showing planet formation carving out gaps in the circumstellar disk.

Microlensing gave us a hand-full of planets this year, among the most interesting is OGLE-2013-BLG-0341L Bb, a terrestrial-mass planet orbiting the secondary component of a binary system with a projected separation of 15 AU. For the microlensing event OGLE-2014-BLG-0124L, the event was observed both from the ground and from Spitzer, making the first joint ground+space detection of an exoplanet microlensing event. Observing the event from two different perspectives allowed for the distance to the lens to be accurately measured.

In early year-review posts, the hints of a planetary ring system around a planet orbiting 1SWASP J140747.93-394542.6 has been discussed. We now find that radial velocity observations of the star have place limits to the mass of the orbiting body, restricting it to a planet or brown dwarf. The rings do exist, and they extend far enough away from the planet to expect them to form moons.

2014 has been an interesting year. A thousand planets have been reported this year, mostly because of Kepler’s enormous contributions. It’s hard to know what 2015 will bring. RV surveys are continuing, and there are still thousands of Kepler candidates awaiting confirmation.

Slight Hiatus

Uranus

Uranus

I’ve been rather distracted by various things so I haven’t had a lot of time to work on this site. Still, it is worth pointing out a fairly noteworthy find from Kepler, namely the discovery of a transiting ice giant planet with a 700-day period at Kepler-421. This planet is beyond the snowline in its system (the distance from a star where water can condense into icy planetesimals and contribute to the planet formation process) and could even have formed where we see it today. Rings and perhaps moons are a distinct possibility. With migration-dominated planetary systems being the easiest to detect, it is quite fortunate and reassuring to find a fairly “normal” planet.

An Earth-Sized Habitable Exoplanet Candidate

Kepler-186f

Kepler-186f (Artist rendering)

A Major milestone was announced Thursday when NASA unveiled Kepler-186 f, a new habitable planet candidate that is, without a doubt, the most Earth-like extrasolar planet known. This planet could legitimately be habitable. Kepler-186 f is one of five known planets orbiting an M1V star with half the radius and mass of Sol. The planet itself is only 1.11 ± 0.14 RE, which suggests that the planet is probably not a mini-Neptune, however we don’t know its mass. If it is composed entirely of volatiles, it has a mass of 0.32 ME, or on the other extreme, if the planet has a pure iron composition, then it has a mass of 3.77 ME. An Earth-like composition places the mass of the planet at 1.44 ME. The planet is probably on the denser end of this, as a low-density planet of this size would probably not have survived the high-XUV stage of the M dwarf’s youth. The other four planets in the system are all less than 1.4 RE, and are likely terrestrial themselves.

The planet gets ~32% of the insolation from its star that Earth gets from ours, which seems a bit on the low end, but there are a couple factors to keep in mind. Because M dwarfs are redder, and because atmospheres scatter blue light, an Earth-like atmosphere for Kepler-186 f would scatter a lower fraction of starlight than Earth’s atmosphere. The insolation from the M dwarf, by virtue of its redder spectrum, gives more heating to a planet than for the same insolation from a G type star. Furthermore, for an Earth-like composition, the planet has a slightly higher mass and could therefore attract a thicker atmosphere, providing more greenhouse heating.

The Planetary Habitability Laboratory lists Kepler-186 f at a rather dismal 17th out of 21. It’s curious they would rank it so pitifully. All of the habitable planet candidates listed as being more Earth-like, with perhaps the exception of Kepler-64 f, are likely low-density mini-Neptunes. Clearly the PHL habitability ranking algorithm needs to be revised. Furthermore, some of the planets listed aren’t even known to exist. Gliese 581 g itself has been effectively disproven.

PHL

PHL

It’s still possible to imagine a feasibly attainable next step: an Earth-sized planet in the habitable zone of a sun-like star — a true Earth analogue. But this is definitely a remarkable discovery and one that probably won’t seem to be outside the realm of habitability when those future discoveries do come about.

At the risk of sounding pessimistic, this may be the only known terrestrial habitable exoplanet candidate we know of. Kepler-64 f could still be a mini-Neptune. Gone are the days where a 5 Earth-mass planet receiving similar insolation as Venus can stir the imagination with the prospects of luscious fields of green, with kittens playfully swatting at butterflies. As the time approaches where we begin to focus our attention on characterising the atmospheres of habitable planet candidates and searching their spectra for biospheres, we will have to prioritise the planets we look at. The overwhelming majority of the planets currently making people’s “habitable planet candidate” list simply aren’t going to be on the receiving end of that kind of attention. The discovery of planets like Kepler-186 f in the solar neighbourhood is the major next step for searching for an extrasolar biosphere.

Staying Relevant

Mildly out-of-date computer.

Mildly out-of-date computer.

It has been nearly 20 years since the discovery of the planet orbiting 51 Pegasi. What followed over the rest of the late 90s were the landmark discoveries of the first eccentric giant planets at 16 Cygni B, and 70 Vir, and the first two-planet system at 47 Ursae Majoris. As new discoveries are made that push the boundary of what is known, prior ones fade into distant memory.

The public interest in these objects also varies with time. It seems odd to think it today, but in the early 1800s, 61 Cygni was wildly more popular than Alpha Centauri. This was merely because at the time, only the former’s distance had been measured, but there does seem to be a correlation between the public interest in an object and its scientific importance. Consider for example three landmark discoveries, the first planet orbiting a sun-like star, the first confirmed brown dwarf, and the first known transiting planet (with stellar hosts 51 Pegasi, Gliese 229 and HD 209458, respectively).

Trends of interest in three landmark discoveries

Trends of interest in three landmark discoveries

51 Pegasi becomes wildly famous, and rightfully so being the first of its kind known. Even today most people with a casual interest in astronomy know why 51 Pegasi is important. Gliese 229 has never really reached the prestige of 51 Pegasi — brown dwarfs just aren’t as exciting, and as time went on, interest faded. What started out as just another hot Jupiter became the most important when it was found to transit, and interest in it has continuously increased over the timeframe allowable to me by Google Ngrams.

As time went on, new planets stopped grabbing people’s attention unless they were set apart by some level of spectacularity. From memory alone, what do you know about the planet HD 290327 b? If you’re like me, absolutely nothing. Still, over time new planets and planetary systems were announced that were genuinely interesting. At the turn of the century, the first super-Earths at Gliese 876 and 55 Cancri held our attention for a while, followed by our first transiting Neptune-mass planet at Gliese 436. HD 69830 and HD 40307 gave us our first multi-planet systems made up of sub-Jovians in the mid-to-late 2000s. CoRoT broke ground with the first transiting super-Earth at the end of the decade and a multi-planet system was imaged at HR 8799.

Throughout this evolution of the kinds of things that have kept our attention, it is truly remarkable to pause and realise how numb we seem to have become to some discoveries. The discovery of Earth-sized planets now occurrs so often that it does not even raise an eyebrow anymore. The time between when a type of discovery goes from immensely exciting to just-another-day-at-arXiv seems to be only on the order of a couple years or so. It almost appears that there seems to be a sort of Moore’s Law at hand for extrasolar planet discoveries as there is with computers.

Earlier this month, the Kepler team made public about 700 new planets. Keep in mind we only just recently achieved a total of a thousand known planets. Now we’re knocking on the door of two thousand known planets. These planets are all in multi-planet systems, which is the foundation of the statistical argument used to validate their existence — a single transiting planet candidate can be any number of false positives, but having multiple candidates in a system is much harder to emulate by a non-planetary phenomenon. Many of the planets are Earth-sized and super-Earth sized, with considerable gains in transiting Neptune-sized planets.

New Kepler Planets

New Kepler Planets

To further drive home the point, among the new Kepler planets are four new habitable planet candidates (at Kepler-174, Kepler-296, Kepler-298 and Kepler-309). At least that’s what they’re being called — it is my assertion that their radii are much more consistent with being low-mass, low-density “mini-Neptunes” or “micro-Jovians.” The combined interest in these four new habitable zone planets is less than half the public interest in Kepler-22 b, for example.

Much closer to home, RV studies on M dwarf stars have yielded eight new planets in the solar neighbourhood, and constrained the frequency of planets around M dwarf stars.

According to our results, M dwarfs are hosts to an abundance of low-mass planets and the occurrence rate of planets less massive than 10 M⊕ is of the order of one planet per star, possibly even greater. …

They, too, report new habitable planet candidates, but their minimum masses are, again, consistent more with being more closely reminiscent of Neptune than Earth. Regardless, it is my opinion that this is actually more interesting than the 700 new planets from Kepler. By now, we know that planets are common. The Galaxy is drowning in planets and while new planets are great for population statistics, individual planet discoveries don’t count for anywhere near what they used to. We are moving from an era of having the attention and focus on planet detection and discovery to an era of planet characterisation. We’re hungry for planets that are actually accessible to HST, Spitzer, Keck and soon(-ish) JWST for transmission spectroscopy and eclipse photometry. New planet discoveries in the solar neighbourhood count for far more than Kepler planets because the nearby planets are the ones that we have a shot at studying in-detail from direct imaging in the near future.

They also report the existence of a Neptune-mass planet in a fairly circular, 400-day orbit around Gliese 229, bringing perhaps a little more relevance and attention to a star that saw its moment of fame twenty years ago.

2013 Review

HD 106906

HD 106906 b, a directly imaged planet announced in 2013

First, foremost, and perhaps most painfully: α Centauri Bb may not really exist. What we thought was the Keplerian signal of an Earth-mass planet at our nearest neighbouring system may actually be noise in the data. While a bit painful, this is how science works – claims are rigorously tested and beaten tirelessly until they either continue to stand on the merit of the evidence, or they are refuted and disproved. This is how we keep the muck out of our pool of knowledge. Stay tuned… this could take a while to fully resolve.

The year began with direct imaging news: A new HST detection of Fomalhaut b (see here), suggesting the “planet” orbit is either not coplanar with the system disk or crosses the ring orbit and has a much lower mass than initially suspected. The imaged planet around β Pic b has also been independently confirmed. A circumbinary planet at 2MASS J01033563-5515561 became the first to be directly imaged. A planet with a mass of ~4 MJ became the lowest-mass planet directly imaged at HD 95086 (with the caveat that it isn’t clear what the nature of Fomalhaut b is). A planet perhaps of similar mass was later reported at GJ 504 (59 Vir).

Habitable zone discoveries started with the first known transiting Jupiter-sized planet in the habitable zone, PH2 b. Then things got very interesting with the simultaneous announcements of a super-Earth straddling the inner edge of the habitable zone of Kepler-69, and two habitable planet candidates at Kepler-62, which was covered here. HARPS found a nice system of planets around the M dwarf GJ 163. One of the planets is somewhat near the habitable zone, but it is my position that this planet does not deserve the attention worthy of a habitable planet candidate, with the planet receiving 40% more irradiation than Earth, and with the host star being an M-type dwarf, the atmosphere will not provide as much scattering of irradiation as Earth’s (Rayleigh scattering is increasingly efficient with decreasing wavelength), causing the surface of the planet to actually receive more than 40% more irradiation than Earth. Despite this, HARPS did provide us with another potentially exciting habitability result, with no less than three super-Earths in the habitable zone of GJ 667 C, with evidence for at least six, perhaps seven total planets there, however a reanalysis of the RV data seems to suggest that these new planets do not exist. Stay tuned…

Other noteworthy announcements included DW Lyn b, a giant planet orbiting a pulsating subdwarf B-type star. A hot Jupiter was also found orbiting a late-K/early-M dwarf by SuperWASP – a particularly rare find. A pair of super-Earths were found in a 2:3 resonance at HD 41248. A giant planet was found in a close orbit around a red giant branch star. Evidence of a second planet accompanying a newly discovered debris disk was presented for κ CrB. A super-Earth around HD 97658 was reported to be transiting (as was suspected two years ago but later dismissed due to a non-detection). The pair of planets at HIP 11952 ended up not existing – an error in compensating for the radial velocity of the observing site relative to the star.

Kepler results continued to stream in, starting with a rather interesting three-planet system at Kepler-68, with a mini-Neptune closest to the star, then an Earth-sized planet just outward of that, and a Jovian planet in a long-period orbit. It was shown that systems of multiple, low-mass planets uncovered by Kepler, like our own solar system, have orbits that are well-aligned with their host star’s equator (see here and here). Kepler results also uncovered a system with a pair of planets in a 2:1 resonance producing very strong transit timing and transit duration variations. A hot Jupiter at Kepler-76 provided strong evidence of super-rotation in the atmosphere via its secondary eclipse visible light photometry. Of particular note is the announcement of a planet smaller than Ganymede(!) at Kepler-37. A new population of small, rocky worlds in extremely short orbits was uncovered by Kepler, specifically Kepler-78 b wih its 8.5 hour orbit and KOI-1843.03 with its 4.2 hour orbit(!). Furthermore, Kepler unveiled the first transiting planets in an open cluster, NGC 6811.

Of particular note is the discovery of a transiting hot Jupiter orbiting a young, oblate, gravity-darkened T Tauri star. This remarkable system seems to imply that the formation mechanism behind hot Jupiters is fairly fast.

Exoplanet catalogues for WASP and Kepler saw their first triple digit identifiers, with WASP reaching WASP-100 and several Kepler planets being assigned triple digit Kepler-ID’s as well (e.g., Kepler-114, Kepler-128, Kepler-177, …).

While Kepler suffered another reaction wheel failure, effectively ending its primary mission, the year ended on a positive note with the launch of Gaia, which will likely find as many planets as Kepler, but in more intermediate period orbits and closer to the solar system.

Gaia Launches Successfully

Gaia launch

Gaia launch

At 6:12 am local time on 19 December, 2013, a Soyuz rocket carrying ESA’s Gaia spacecraft launched from French Guiana. Gaia was successfully sent into a solar orbit, at the Earth-Sun L2 point.

And so begins a five year mission to map the Galaxy. It’s hard to know for sure what discoveries Gaia will make, but it is clear that we will learn a great deal about the Galaxy.