TOI-5205 b: A Forbidden Planet?

Posted on February 20, 2023 by Shubham Kanodia

The Discovery

For the past four years alongside the HPF survey to discover new exoplanets, we have also been using HPF to follow-up on planet candidates discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS) mission. The TESS survey is observing the entire sky every two years, and has already found thousands of planet candidates, many of which orbit M-dwarfs.  By observing millions of M-dwarfs, it is able to find giant transiting exoplanets (six times Earth’s radius or larger) around M-dwarf stars, which are supposed to be quite rare. The impact of this mission on our sample of these planets can be seen below, where up until 2018 we knew of just four of these planets, while there are now around 15 of them.

Discovery of confirmed giant transiting exoplanets orbiting M-dwarfs. The huge impact of TESS and our follow-up efforts utilizing HPF and NEID can be seen with the increase in discoveries post 2019.

TOI-5205

As part of these follow-up efforts, we began observing one of these planet candidates–TOI-5205 b–with HPF and other ground-based observing resources. This candidate seemed particularly interesting since it orbits a mid-M dwarf that is just about 40% the size and mass of the Sun, and about 3400 K in temperature (compared to the 5800 K for the Sun). This is an especially small host star when compared to all the previous giant Jupiter-sized planets, which were found around early M-dwarfs (roughly 60% Solar mass and 4000 K in temperature).

We started the  characterization of this planet candidate with ground-based transits from a multitude of telescopes, including the 3.5-meter ARC telescope (ARCTIC) at Apache-Point Observatory, USA, 0.6 m Red Buttes Observatory (RBO) in Wyoming, USA, and 0.3 m TMMT telescope in Las Campanas, Chile. This was important because the host-star (TOI-5205) had another star present about 0.1% of a degree away on the sky, which contaminates the low-resolution TESS images. These ground-based transits helped us estimate the level of this contamination and also refine the transit timing for the planet.

Different ground-based photometry obtained for TOI-5205 b. The transit models in red were not used for obtain the transit depth due to various systematics, but just to refine the transit timing.

 

The first of these transits on April 22, 2022 with APO confirmed that the initial contamination efforts were close, and we did indeed have a massive Jupiter-sized object orbiting a mid-M dwarf. This transit, at nearly 7% brightness reduction, is one of the deepest known transits for exoplanets orbiting main-sequence stars!

Artist rendition showing the relative size of the TOI-5205 b system compared to similar Jupiter-like planets around Solar-type stars (Credit: Katherine Cain, Carnegie)

 

Subsequently we obtained seven HPF radial velocities (RV), which helped us estimate the mass of this object to be planetary in nature, and in fact almost exactly that of Jupiter (~ 1.08 Jupiter masses). TOI-5205~b turned out to be the first confirmed Jupiter-type planet such a low-mass star.  The ratio of the planet’s mass to that of the star is about 0.3%, which one of the largest for all M-dwarf planets.

 

TOI-5205 b with respect to the other massive planets orbiting M-dwarfs. Its red marker shows how its host star is much lower in mass compared to the other similar systems.

 

So why is TOI-5205 b forbidden?

Core Accretion

For context, a 0.3% mass ratio object around a Solar-type star would have a mass of about 3 Jupiter masses; at these masses, the conventional core-accretion theories of planet formation start to have a harder time to form these objects. Under this core-accretion paradigm, planets must first form a solid core that’s primarily made of heavy elements (not hydrogen/helium) and roughly 10 Earth masses. Once it reaches this threshold, the planet undergoes a runaway process of gas accretion where it quickly builds up a massive gaseous envelope  reaching Jupiter in size and mass (see here for a review).

So far all the Jupiter-like planets have been around early M-dwarfs, where the primary challenge has been forming this initial heavy-element core before the protoplanetary disks in which these planets form evaporate away. This was first shown by the seminal papers from Gregory Laughlin, and also Ida & Lin, which suggested that these low-mass stars would struggle to form these massive-enough cores in a timely fashion before the disk vanishes because the host-star is lower in mass (flashback to Kepler’s third law, where the orbital period is inversely proportional to the host-star mass). One way around this was the idea of gravitational instabilities in turbulent disks forming planets.

Gravitational Instability

Under the gravitational instability scenario proposed by Alan Boss, massive disks (typically assumed to be > 10% the mass of the host-star) tend to form self-gravitating clumps of dust and gas far away from their host star where the disk is quite cool. These clumps start off in spiral arms (similar to the spiral arms of the Milky Way Galaxy), and eventually coalesce to form planets. The main reason this presents a viable alternative to core-accretion is that this is a very fast process, where planets can form quickly in a few thousand years (as opposed to the millions of years it takes through core-accretion).

However, the problem for TOI-5205 b orbiting a mid-M dwarf is not just about the timescale of formation in the protoplanetary disks, but more of mass budgets.

Mass Budgets

It is typically assumed that planet formation begins in Class II protoplanetary disks which are about 1 – 10 million years old. These disks can now be studied with facilities like the Atacama Large Millimeter/sub-millimeter Array (ALMA) in the Atacama Desert, Chile which consists of  66 individual radio telescopes, and enable estimates of the amount of dust and gas present in a star’s protoplanetary disk.

ALMA: Large array of radio telescopes in Chile (Credit: ESO)

Studies of disks suggest that they tend to be quite diverse in their properties, while also following an approximate relation where the disk masses are proportional to the host-star mass. For stars similar to TOI-5205, these relations suggest that they have about 10 Earth masses of dust (here and here), albeit with a large spread in the predictions from these measurements.

For comparison, models of planetary interiors utilize our understanding of how gases and metals react to high temperatures and pressures to estimate what the interiors of these planets look like (here). Put simply, based on the planet’s mass, radius, temperature, etc., they can predict how much metal (or non hydrogen/helium heavy elements) they have. These models also have a large scatter in their predictions, but on average suggest that a planet such as TOI-5205 b should have about 60 Earth masses of heavy elements inside it.

And herein lies the problem: even if the entire protoplanetary disk collapsed to form one big ball of heavy elements, it still falls short by about a factor of 5! That being said, we know that TOI-5205 b exists, therefore there is some gap in our understanding of these disks, or planetary interiors, or the process of planet formation (or the most likely scenario, all of the above!).

Possible Solutions

There is the possibility that the ALMA observations are underestimating the disk dust masses, and that these disks indeed have a lot more dust in them. A few studies have shown that this might be the case, especially if the dust is locked up in particles that are larger than the millimeter regions probed by ALMA (see here for a review). If this were the case, then there would be a lot more material available to form these massive planets, which would help resolve the discrepancy presented above.

Furthermore, studies have suggested that planet formation is already underway for Class II disks, especially when it comes to giant planets. This implies that the total mass budget available for forming planets is not what is observed by ALMA for such disks, but perhaps much earlier during their Class 0 or I phase, when the disks are more massive by almost an order of magnitude!

 

The evolution of protoplanetary disks. Typical observations with ALMA are during the Class II phase. However, it is now starting to be established that planet formation begins much earlier.

 

Lastly, some of the assumptions made by some of these planetary interior models have not stood the test of time. Recent observations of the Solar-system giant planets with the Juno and Cassini missions have shown that their interiors are quite complex, with diffused cores where the helium and metals are immiscible in metallic hydrogen (see here and here for a review). If this were true for giant extrasolar planets as well, then these models would be overestimating their heavy-element content.

 

Conclusion

The existence of TOI-5205 b can perhaps be attributed to a combination of factors suggested above, which suggest the need for revisions in our understanding of protoplanetary disks, planet formation, and also planet interiors. Follow-up efforts such as those conducted by HPF for Giant Exoplanets around M-dwarf Stars (GEMS) are already hinting at the presence of more such planets, which suggests that TOI-5205 b–while definitely an outlier–isn’t the only one. If so, how frequently do these forbidden planets form?

 

You can read more about this intriguing system in this manuscript from HPF team member Shubham Kanodia.

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TOI-2136b: Habitable, but not to us?

Posted on September 15, 2022 by Paul M Robertson

Will Life on Other Planets Look Like Ours?

Finding life somewhere other than Earth would be a truly monumental discovery, but will we know it when we see it? For as long as astronomers have been studying the stars, they have imagined far off worlds, usually something like Earth. And that’s why exoplanet scientists covet the “Earth 2.0,” an exoplanet just like Earth, that may just house the life we have long sought. But that does not mean that scientists aren’t also thinking about and looking for life in unusual places…

TOI-2136b: Characterizing the Planet with HPF, TESS, and ARCTIC

Scientists working with HPF–led by Science Team member Corey Beard–have recently characterized the transiting exoplanet, TOI-2136b. First identified by the transiting exoplanet survey satellite (TESS), we then utilized the diffuser-assisted ARCTIC instrument to refine the size of TOI-2136b. We learned that it is a sub-Neptune, meaning that a huge part of the planet is a big, gaseous atmosphere, just like Neptune, but with a radius only twice as big as Earth. Using HPF, our group was able to constrain the planet mass by measuring the Doppler wobble of the system. As a result, we confirmed that this system is probably a gaseous, puffy planet, rather than a rocky planet like Earth.

TOI 2136b Transits

Left: Transits of TOI-2136b observed with TESS. Right: Transit of TOI-2136b taken with the diffuser-assisted ARCTIC instrument. By measuring the brightness of a star every two minutes, we are able to identify periodic dips that suggest a planet is passing in front.

A Cold Haber World?

Even more fascinating, TOI-2136b just might have an environment amenable to an exotic kind of life we’ve never seen on Earth. Orbiting its star once every 7.8 days, TOI-2136b is a lot closer to its star than the Earth is to the Sun. Luckily, TOI-2136b’s host star is an M dwarf, a cooler, redder star than the Sun. As a result, the equilibrium temperature of TOI-2136b is “only” 129o C (265o F)… way too hot for humans to survive. However, because the planet is in a size regime where exoplanets most likely have large atmospheres of hydrogen and helium, the pressure beneath the atmosphere is likely to be very high: so high, in fact, that water–which would normally have boiled into steam–can be forced to stay in a liquid state. This is highly significant, as liquid water is essential for any currently known kind of life.

Scientists originally proposed the idea of a Cold Haber World in 2013, a unique environment where bacteria live inside of a planetary ocean surrounded by a thick gaseous atmosphere. On Earth, some industries use what we call the “Haber process” at extremely high temperatures to produce ammonia by combining H2 and N2. Bacteria on TOI-2136b could combine hydrogen and nitrogen from the atmosphere using this same Haber process, though the temperature would be lower in the planetary environment than in the lab on Earth (hence the use of the word “cold”). Much like photosynthesis on Earth, these bacteria would get energy from the process, making it an appealing way for bacteria to survive.

But so what? How would we ever know that this was happening? Well, it happens that TOI-2136b has the perfect properties to make it viable for biosignature searches. At only twice the size of Earth, TOI-2136b is large enough that it probably has a thick atmosphere of hydrogen and helium, but small enough (<3.75 Earth radii) that it can’t produce ammonia abiotically; that is, without the help of life. With a temperature of 129o C, the planet can have liquid water due to the high pressure of a thick atmosphere. And the cherry on top? This system is close enough to Earth that we could look at it with the James Webb Space Telescope (JWST), and actually figure out what its atmosphere is made of! As a part of our analysis, our team simulated what a JWST observation with NIRSPEC–one of the spectrometers on JWST–would look like, and the results are exciting. Clear molecular features from water, carbon dioxide, and methane are visible for any mass of the planet allowed by our HPF observations. Our simulations don’t include any ammonia lines, but that is because this planet shouldn’t be producing ammonia without any bacteria. So, if we observe this system and detect it, that would be compelling preliminary evidence that there is life on the planet, using this Haber process to create energy for itself.

TOI 2136b NIRSPEC

By using our knowledge of chemistry and physics, we are able to simulate what a JWST NIRSPEC spectrum of TOI-2136b might look like. While the planet transits the star, its atmosphere absorbs some of the light. By looking at what colors are absorbed, we are able to determine which chemicals exist in the planet’s atmosphere.

While we haven’t detected any biosignatures yet on TOI-2136b, keep an eye out. Other groups are already working on studying this system more, and it has a lot of potential!

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Does it rain on vB 10?

Posted on November 25, 2021 by Shubham Kanodia

The primary science goal of HPF is to find planets around mid-to-late M dwarfs using the radial velocity method. The largest of these are about a third the size of the Sun, and about 2/3rd as hot (~ 3000 K vs 5777 K for the Sun). To achieve this, we conduct routine and intensive observations of a small (carefully selected) group of such stars, one of which is the ‘ultracool-dwarf’ vB 10.

So what is vB 10?

Discovered in 1944 by van Biesbroeck (hence the ‘vB’ in vB 10), it was the faintest star known back then and was observed using photographic plates. Interestingly enough, vB 10 was discovered using observations from 2.1 m telescope at McDonald Observatory, which now also hosts the 10 m Hobby-Eberly Telescope where HPF is situated. It’s about a tenth the radius of the Sun, (in fact is closer to Jupiter in size), while being about half the temperature of the Sun at 2800 K. An M8 dwarf, it belongs to the category of objects referred to as ‘ultracool dwarfs’, and is pretty much the smallest star that can still fuse Hydrogen (1H) to Helium (stars less massive than M8 dwarfs are typically called brown dwarfs; Figure 1). Over the years, there have been numerous claims and dismissals (here and here) of planetary detection around vB 10, which pretty much rule out giant gaseous Jovian type planets around this star.

Figure 1: Size comparison of the Sun, low mass stars, brown dwarfs, and planetary bodies. Image Credit: STScI

What did we find with HPF?

With HPF’s near-infrared wavelength coverage, stable instrument, and HET’s large mirror, we are trying to find smaller planets around vB 10, which have so far been out of reach of the previous generations of planet-hunting instruments. Aside from obtaining precise radial velocity measurements of the star, these observations are also useful for serendipitous observations of transient phenomena.  HPF observed vB 10 on 2019 August 20th at 05:40 (UTC), as part of its regular queue operations; subsequent analysis a couple months after  revealed a huge flux increase (emission) in some of the atomic lines present in the near-infrared spectra (Figure 2). These are indicative of a stellar flare on vB 10, a phenomena that has been observed in the past (here, here and here).

Figure 2: The top panel shows the excess flux at ~ 8500 Angstroms, which is a Calcium atomic transition, compared to the template (average) spectrum in the middle panel. The bottom panel shows the difference between the two, highlighting the excess flux that was measured on this date.

What are flares?

Stellar flares are short lived intense eruptions of energy on stellar surfaces, with their exact provenance not being completely understood. The current best hypothesis is that when a magnetic field lines (think of them as a tense and taut rubber band) on stellar surfaces rupture and reconnect, they release a LOT of magnetic potential energy. Some of this is converted to thermal energy, and generates very high temperatures, while the rest accelerates ions and electrons on the star to very fast speeds. Some of the gas at the site of magnetic reconnection rushes towards the surface of the star, and the rest is shot above the flare away from the stellar surface (Figure 4).

Figure 3: A schematic of a flare showing the magnetic field lines in green, and energy outflows. The red circles represent over-densities of plasma or hot gas caused by the release of energy at the reconnection site. Image Credit: Liu et al. (2008)

 

A Red Excess?

Apart from the Calcium lines, we observe a few different atoms that were excited enough (in a high energy state), that they displayed emission peaks.  Specifically, with HPF we are able to observe the three atomic transitions (triplet) of Helium at about 10830 A (the same transitions which we use to detect escaping planetary atmospheres). These observations at wavelengths > 9000 A, are typically inaccessible for traditional optical, CCD based spectrographs. For example, we see that the Helium atomic lines (Figure 3), show excess flux similar to Calcium, Hydrogen, etc. However, we also find a red excess which cannot be explained by the atomic transitions.

Figure 4. Emission flux observed in Helium, showing the positions of the triplet, as well as the peaks. When we fit a model to the Helium lines (shown in solid blue, red and purple), we see that it cannot reproduce an excess towards the redder wavelengths (dotted red).

What is coronal rain?

This excess has about 30% the flux of the main Helium lines, and is offset in wavelength by ~ 3 A, which corresponds to a velocity shift of 70 km/s (in other words, the Helium atoms that emitted the red feature were falling with a velocity of about 70 km/s towards the surface of the star). It is indicative of a phenomenon called coronal rain, which has been observed for the Sun (video below) for many decades.

While a fraction of the hot gas that is shot away from the star after the reconnection escapes the gravity of the star and is ejected out as a coronal mass ejections (CME), not all of it is hot enough (and hence moving fast enough) to escape the star. Some of it falls back to the star during the gradual phase, which occurs a while after the initial reconnection event (typically referred to as the impulse phase). This gas falling towards the surface of the star, is moving away from us (ignoring projection effects; this gas  in our line of sight is between us and the star), and is therefore red-shifted compare to other atoms present in the stellar atmosphere. If it is hot and dense enough, it can emit light while falling towards the surface of the star. This is coronal rain is what we think is responsible for the red asymmetry we see in our Helium observations during the vB 10 flare — hot, dense blobs of gas that are falling towards the surface of the star, from high in the stellar atmosphere, after they got there typically due to the excess energy emitted during stellar flares. In the video below we see an actual resolved video of the Solar surface showing coronal rain with the hot blobs of gas falling towards the Sun along the curved magnetic lines.

Since we cannot resolve the surfaces of distant stars, studying these asymmetries in the atomic lines tells us about the movement of the atoms that were emitting the photons we detect. It helps us understand not just the composition of stars, but also their structure and velocity fields. While there have been previous observations of some early M dwarfs which allude red asymmetries to coronal rain, this is the first quantitative analysis for such a cool, late-type star, and also the first using the Helium atom as the indicator.  All the clues suggest that it does rain on vB 10… (though not the kind of pleasant H20 rain we’re used to, but in the form of extremely hot blobs of gas at ~ 10,000 Kelvin).

Additionally we’re able to use images from the guide camera accompanying HPF observations to place limits on the total amount of energy emitted by the star during this flare, and also place limits on its duration and frequency. Thereby this work presents a unique combination of high resolution spectroscopy + photometry to study the interesting phenomenon of stellar flares, and the features accompanying them. You can read more about this in this recent research article by HPF team member Shubham Kanodia.

 

 

 

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