Is there a hidden repository of systems with multiple planets?

Is there a hidden repository of systems with multiple planets?

Is there a hidden repository of systems with multiple planets?







				

Kepler has dead, long life to Kepler! The Kepler spacecraft has transformed our knowledge about the exoplanets and it is therefore very sad to have to say goodbye. Kepler has had a special relevance in exoplanetary systems of the size of Neptune or Earth, specifically in those very close in those planets and with orbital periods less than a hundred days. The combination of transits with studies of radial speed they have allowed discover thousands of planets and therefore understand the demographics of exoplanetary systems.

For example, we have learned that there is a robust correlation between the metallicity of the host star and the presence of hot Jupiters, which may be evidence for the planetary formation model of core accretion. Strangely, in multiple systems the rocky planets usually appear with a wide range of metallicities. Is it possible that this is because there is not enough material in the protoplanetary disks of low-metallic stars to feed the planets of Jovian masses? Or is it because we have observational biases? Radial velocity studies tend to avoid stars with low metallicity since it has few absorption lines that can be measured; Are the little metallic stars well sampled? And what is even more curious, the systems with hot Jupiters are usually very different from the compact systems with several planets. To try to solve this puzzle, the authors of today's article try to shed light on the relationship between the types of exoplanetary systems and the metallicity of the host star.

Today's article


The authors use high-resolution spectra taken with Keck HIRES, a spectrograph that has found several planets in recent years. Using the absorption lines between 5160 and 7800 angstroms, the authors delimited the abundances of 15 different elements in the photospheres of the stars matching the spectra obtained with spectral models generated by Spectroscopy Made Easy algorithm. Then they screened the planets data using the  NASA Exoplanet Archive and defined three types of systems of interest (Table 1).

Table 1: Exoplanet system categories



























Type of system

Definition of the authors

 Number of systems



Hot jupiters
It has planets with a mass> 0.5 Jupiter masses or a radius> 0.75 Jupiter radii and a semimajor axis <= 0.3 AU
 104


Cold Jupiters
Same as hot Jupiters but with a semimajor axis> 0.3 AU
 87


Compact multiplanetarios
> = 3 planets to <1 AU
 105




The only overlap between these categories was a system that shared the category of multiple system and hot Jupiters and nine systems that shared the category of having more than three planets and cold Jupiters.

The authors build a probability density function for these three exoplanetric systems based on a host star that was abundant in iron. The empirical data are discrete and noisy and have had to interpolate and smooth the distributions using an estimator called  Gaussian kernel density  (This technique is similar to that used to extract weak exoplanet traffic signals from noisy photometric data.) In fact, one of the authors of the article wrote a code called George to do just that.) Finally, they found confidence intervals repeating and randomly resampling their data. (Or what is the same thing using the technique of bootstrapping. See Figure 1)





Fig. 1: Probability density function of the three categories of exoplanetary systems after using an estimate of Gaussian kernel density. The x axis is the relation [Fe/H], which is a logarithmic way to measure the relationship between iron and hydrogen (also called metallicity). Note how hot and cold Jupiters tend to be present in stars of greater metallicity. In contrast, the presence of multiple compact systems is weak with respect to metallicity except for the final part of the diagram. Figure 1 of the article.



 

However, we can only measure stellar abundances in the star photosphere. What happens if, as the models suggest, the heavy elements end up dissipating inside with time? Are the stars with several planets and spectra poor in metals simply older and not poor in metals? To verify this, the authors drew the abundance of alpha elements such as Silicon (Si) versus iron (Fe). If the stars really have inherently low abundances of Fe, then we should expect to see a higher Si / Fe ratio. This happens because of the different temporal scales of two types of supernova classes that expel different elements. (More details here. Here we can see, in Fig.2 that the final end of the distribution curves of poor in Iron goes up.

conclusion

As can be seen in Fig. 1, the metallicity of the host hot Jupiter star is maintained. Interestingly, cold Jupiters tend to correlate with their warmer counterparts. This can give us a clue: any mechanism of metal-dependent formation that generates hot Jupiters can also allow the formation of planets the size of Jupiter beyond.





Fig. 2: Ratio of silicon alpha element to iron, depending on iron. The authors draw attention to the empty area in the upper right corner and that this may be related to the stars in the area of ​​the galactic thick disk which is less observable because they are more tenuous. Figure 2 of today's article.



Another article, published almost simultaneously to this, also found that giant planets tend to form near metals-rich stars. Furthermore, these authors conclude that the probability that the planets are formed in the first place can also be a function of metallicity.

The fact that multiple compact systems can be formed around stars that have a range of metallicities is maintained, with an interesting change in the trend at lower metalities, -0.5 <Fe / H <-0.3, where it seems that the probability. Is there an excess of systems with multiple undiscovered planets that lie undiscovered with an even lower metallicity? Even more ancient stars in the Milky Way with Metalicities of Fe / H = -3.5 and even less. Unfortunately, Kepler did not have the opportunity to say a few last words about this and we will have to wait for the next generation with more precision in the estimation of radial speed to be able to study the poor stars in metals.

 



				



				

							
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