Transit Photometry: Questions about Detecting Exoplanets

[le chat]

Hello everyone,

As my first post in this forum, i want to ask two questions, mainly out of personal interest, concerning the possibilities of transit photometry in detecting exoplanets:

1. For larger planets or planets orbiting farther away from the host star, the measured drop of intensity during primary eclipse should be more drastic since the planet blocks more of the stars emitted flux as seen by an observer on a distant planet (e.g. Earth ;)). Is it possible to make similar connections for the duration of the measured decrease in brightness? More precisely, can we infer, solely from measuring the transit duration, something about some orbital features of the planet? (other than the presence of other massive bodies if aperiodicities in the brightness drop are observed over time and not regarding the combination with other detection methods such as radial velocity)
2. I have picked up somewhere that planets orbiting farther out are more difficult to detect with the transit method. To me this is not entirely conclusive as i would expect more distant planets to block a greater fraction of the stars light since it's angular diameter as seen from Earth is larger at primary eclipse in comparison to a planet of the same size orbiting closer in! Thus the question: Does this statement refer to the fact that planets farther out are less likely to have an inclination close enough to 90° for a transit to happen along the line of sight meaning that it is statistically less likely for farther out planets to be detected via this method? Or is there really another issue with planets orbiting farther out?

Thank's in advance for your answers!

 

[phyzguy]

The stars are so far away that light rays reaching the Earth from distant stars are essentially parallel. This means that a transiting planet will block essentially the same amount of light whether it is close to the star or far from the star. Said another way, the angular diameter of a planet orbiting another star is essentially the same regardless of its orbital diameter. Consider two planets, one orbiting a distant star (100 light years away) in an orbit with a radius of 0.1 AU and one orbiting with an orbital radius of 10 AU. The distance from the Earth to the two planets differs by only .00015 %. So the reason that planets orbiting farther out are more difficult to detect with the transit method is that planets farther out are less likely to have an inclination close enough to 90° for a transit to happen along the line of sight, as you said.

 

[le chat]

Thanks for your answer! Makes sense that the angular diameter change is negligible considering interstellar distances. Please allow me to pose a follow-up question on the subject: In the mean time i also explored a bit the statistics on exoplanets.eu. I am trying to make sense of the statistics found when plotting planetary radius against orbital period (see attachment). One can clearly see two distinct regions where the number of detections accumulate. This is in the upper left corner (large radii, small preiods) and in the lower left region. The former is probably where the transit method works most efficiently, since large planets block more light and smaller periods allow for quicker confirmation by measuring over several periods. My question would be, what causes the high density of detections in the lower part? Are theses detections via radial velocity? Also why is there a region in between the two denser regions where only a few planets where detected?

 

[phyzguy]

Bottom line - I don't know. I notice on this website that you can actually restrict the detections to just transit detections (the green "Detection' pull-down), and it still looks basically the same, so the reason is not what you said.

 

[Drakkith]

My question would be, what causes the high density of detections in the lower part?

My guess is that there are simply a lot more planets of small mass compared to planets of large mass.

Also why is there a region in between the two denser regions where only a few planets where detected?

Good question. I'd like to know the answer to this myself.

 

[ExoExplorer]

Thanks for your answer! Makes sense that the angular diameter change is negligible considering interstellar distances. Please allow me to pose a follow-up question on the subject: In the mean time i also explored a bit the statistics on exoplanets.eu. I am trying to make sense of the statistics found when plotting planetary radius against orbital period (see attachment). One can clearly see two distinct regions where the number of detections accumulate. This is in the upper left corner (large radii, small preiods) and in the lower left region. The former is probably where the transit method works most efficiently, since large planets block more light and smaller periods allow for quicker confirmation by measuring over several periods. My question would be, what causes the high density of detections in the lower part? Are theses detections via radial velocity? Also why is there a region in between the two denser regions where only a few planets where detected?

Very interesting question. This planet population trend can also be found in NASA Exoplanet Archive, and it is not observational biases! It is actually an unresolved phenomenon that is not predicted by current theories of planet formation. This gap has many names, such as sub-Jovian desert, super-Neptunian desert, and sub-Jovian Pampas. It lies approximately between Neptune-size and Jupiter-size planets representing the most underpopulated planet desert. It was first described by Szabó and Kiss in 2011. Later, the desert was also found to appear in radius distribution as Kepler collected more data (Beaugé & Nesvorný, 2012). The lack of well-characterized planet in this region prevents us from further assessing the evolution and formation of these planets. One possible explanation is photon-evaporation (Kurokawa & Nakamoto, 2014). In this scenario, the atmosphere of close-in sub-Jupiters and super-Neptunes experiences intensive escape induced by stellar radiation and wind leaving behind the small cores, which are either super-Earth- or mini-Neptune-size. The photon-evaporation model successfully reproduced the observed trend. Another mechanism is runaway gas accretion (Batygin et al., 2016). A planet with mass higher than 10 M undergoes runaway gas accretion, and the mass and radius of the planet increase dramatically and rapidly surpass the desert region forming a Jovian world. In contrast, a planet with mass lower than 10 M forms a Neptunian world, and the worlds in between Jupiter and Neptune are short-lived and very few in number. We still don't know which of these two mechanisms might be responsible for the formation of desert. It could be the combination of both.