Closely Orbiting Scorched Sub-Neptunes
Since I began working with K2 data back in April, I began to notice that there's an odd and uncommon population of planets that shouldn't even exist. These are sub-Neptunes - planets greater than the 1.6 R_Earth transition into a gaseous world but below the radius of Neptune - that orbit incredibly close to their host stars. I'm not talking about orbital periods lasting a few days like typical Hot Neptunes, Instead, these planets take less than ONE DAY to orbit, with some having a year lasting a handful of HOURS. The most extreme of these, EPIC 203533312.01 (found by Adams et al 2016) is over 3 times the radius of Earth and orbits every 4.2 hours around a 1.15 R_Sun star. Planets this close in are terribly hot, with temperatures usually in excess of 2,500 F (1,644 K). Some of the "colder" planets in this category orbit very close to a red dwarf host, where tidal forces are strong enough to tear a planet to pieces. These two scenarios - hellish heat and extreme tidal forces - pose a problem for our current understanding of planets between 1.6 and 3.5 R_Earth. Why? Because planets in this size range are expected to be gas planets similar to Uranus and Neptune, but the conditions these Closely Orbiting Scorched Sub-Neptunes (COSSNs) face would destroy and gas-rich planet. Lets take the planet candidate EPIC 212432685.01/Thantos, found by Cabbink on Exoplanet Explorers, as an example. It's a 2.02 R_Earth Sub-Neptune orbiting every 0.53 days around a star larger than the Sun, and has an equilibrium temperature of about 2,712 K (4,422 F). If Thanatos were gaseous, the incredibly high temperatures would cause the atmosphere to bloat up, creating a larger planet than we see today. Alternatively, if Thanatos already is puffed up, its mass would likely be less than twice that of Earth. It wouldn't be enough to hold onto that puffed-up gas envelope, and the gas would be blown away by stellar wind, leaving a small rocky core behind. In addition, Thanatos' close proximity to such a large star means that it is under incredible tidal forces, probably much more extreme than what Io gets from Jupiter. A gas-rich planet would be torn to pieces under these forces. Knowing this, there is only one explanation for a planet like Thanatos: an enormous, molten rock-iron planet. Multiple planets like Thanatos have been found by me and other users on Exoplanet Explorers, and while some could be eclipsing binaries, others are very likely to fall into the COSSN category. Currently, how such a planet would form is quite a mystery, as larger COSSNs could be up to 100-200 M_Earth. As of now, I believe the most likely explanation is that these planets are the rocky remnants of evaporated gas giants that formed with larger cores. A great example is Smertrios (HD 149026 b), which has a temperature of 2,300 K (3,680 F) and contains a large rocky core of 80-110 M_Earth. Evaporating the gas layer of Smertrios would create an exceptionally large COSSN, with a radius likely over 3.0 R_Earth. A list of COSSNs can be found below. @ajamyajax @zoo3hans @Shellface do any or your finds fall into this category?
EPIC 206024342.02: 1.97 Re, 0.911657 days, 0.02009 AU, 2,483 K (4,010 F). Found by shutcheon
EPIC 212432685.01: 2.018 Re, 0.53164 days, 0.0131589 AU, 2,712 (4,422 F). Found by Cabbink
EPIC 212157262.03: 1.845 Re, 2.8727 days, 0.039149 AU, 1,192 K (1,686 F). Found by shutcheon. This planet could be rocky due to its host star's high metallicity of +0.25 [Fe/H]
EPIC 203533312.01: 3.11 Re, 0.17566 days, 0.005608 AU, 4,179 K (7,063 F). Found by Adams et al.
EPIC 213919915.01: 2.96 Re, 1.489316 days, 0.028039 AU, 2,407 K (3,874 F). My discovery.
EPIC 210592325.01: 3.6 Re, 0.676 days, 0.01679 AU, 2,853 K (4,677 F). Found by Vidar87.
EPIC 201321030.01: 1.7 Re, 0.1845 days, 0.004644 AU, 1,448 K (2,147 F). My discovery. Planet candidate is under incredible tidal stress from red dwarf host.
EPIC 201675809.01: 3.46 Re, 0.82541 days, 0.01696 AU, 3,206 K (5,311 F). My discovery; weak candidate
EPIC 248410630.01: 3.4 Re, 0.978205 days, 0.01837 AU, 2,228 K (3,550 F). My discovery; weak candidate
EPIC 248455600.01: 2.097 Re, 0.398337 days, 0.009497 AU, 1,587 K (2,397 F). My discovery. Also under high tidal stress
EPIC 248472140.01: 3.07 Re, 0.760053 days, 0.016532 AU, 2,363 K (3,794 F). My discovery; most likely COSSN yet.
EPIC 201119435.01: 3.4 Re, 0.64086 days, 0.012977 AU, 1,449 K (2,148 F). Found by Vidar87. May not be under enough tidal stress to be a COSSN.
EPIC 201595106.01: 1.61 Re, 0.8771 days, 0.018095 AU, 2,108 K (3,335 F). My discovery.
EPIC 201205683.01: 2.38 Re, 0.288993 days, 0.0049714 AU, 998 K (1,337 F). My discovery. Under too much tidal stress to be a gaseous planet.
Yes of course, we find many of the same candidates you and the others find on EE using similar methods -- including many of these interesting possible exoplanets. The trick of course is getting any reasonably good and repeating transit past the contamination checks, and then vetted them with an accepted scientific method as an exoplanet.. If they aren't documented already...
Maybe contact your EE science team and suggest a COSSN focused paper, and ask if they are interested in pursuing this project. Also best to include Dolorous_Edd in any of your PH pings here who is an expert transit finder and very good with stellar data.
Best of luck.
I had recently E-Mailed Ian Crossfield about how to get my planet candidate discoveries officially recognized (as suggested by Al Schmitt), so I'm going to hold onto the idea of a COSSN research paper until he gets back. However, I've been considering to try detecting some of these candidates with the radial velocity method, once I learn how to do so, or search for transits of the more obvious COSSNs with ground-based telescopes.
There's one problem with researching COSSNs: there are so many other planets and system to investigate, such as EPIC 220522664, which may have up to 8 planets orbiting it, or some other interesting multi-planet systems like EPIC 228801451, EPIC 201528828, EPIC 220650843, EPIC 220221272, EPIC 210696763, K2-5 (maybe up to 6 planets), and more. I'm not too sure if I should focus on studying a potential new class of planets or trying to vet the dozens of incredible discoveries found on Exoplanet Explorers.
All right, sounds like you have a plan then. Good luck!
@ajamyajax here’s a truly terrifying COSSN candidate found by maddendd on EE: https://www.zooniverse.org/projects/ianc2/exoplanet-explorers/talk/785/522341?comment=881274&page=1
I need your help sizing the star, because it’s J-H and H-K values are unlike any other I’ve seen before: 0.054/0.146.
Yeah a wide range in the color math do not make this easy.. But having looked at many of these I would say this often indicates more than one star in the aperture and possibly a binary.
Re 247812410 from ProtoJeb's request: this target looks like an early to mid A-type star as indicated by one of the color math combinations, Huber's 9909K Teff value, and the obvious Simbad listing. I didn't get a good look for a planet transit myself, but still possible. Other possible explanations are a background binary of course, quasi-periodic stellar changes in brightness, or maybe even several glitches that repeat at the observed period and amplify the dips.
EPIC, 2MASS, J mag, H mag, K mag, J - H, H - K, (J-H spectral type, stellar mass est) (H-K spectral type, stellar mass est)
247812410 , 2MASS J05001459+2429419 , 10.873 , 10.819 , 10.673 , 0.054 , 0.146 , ('A5V', 1.9) , ('K5V',0.75)
From NEA, K2 Targets within search area:
EPIC Number RA [decimal degrees] Dec [decimal degrees] Distance [arc sec] Kepler-band [mag] Campaign Number
247812410 75.0608 24.495 0.01 11.841 13
247810580 75.0548 24.4821 50.52 15.817 13
247806973 75.0714 24.4567 142.24 15.002 13
GO13122 Howard The Masses and Prevalence of Small Planets with K2 - Cycle 4
Listed as HD 284032 -- Star on Simbad, Spectral type: A, 05 00 14.59 +24 29 42.0
It actually has spectrum, it is A2V star
Otherwise looks pretty lonely (at least no visible very close companion )
Also, I just stumbled across another COSSN candidate around EPIC 248793536, which is about 2.2 R_Earth and orbits a possible K-dwarf every 7.2 hours. Marked on ExoFOP as a possible PC.