Two planets in formation discovered in the HD 169142 system

The HD 169142 system (Image ALMA (ESO/NAOJ/NRAO)/ Fedele et al.)
The HD 169142 system (Image ALMA (ESO/NAOJ/NRAO)/ Fedele et al.)

An article published in the journal “Astronomy & Astrophysics” describes the discovery of traces of two planets that are forming in the young system HD 169142. A team of researchers led by Davide Fedele of the Institute of Astrophysics in Florence, Italy, used the ALMA radio telescope to study the of gas and dust disk surrounding the young star identifying gap rings compatible with the formation of planets similar to Jupiter.

The HD 169142 system, about 470 light years away from Earth, is one of many discovered during their formation phase in recent years. However, each of them shows a different situation with a slightly different age and therefore a more or less advanced stage of planetary formation. As for the number of planets in formation, even using the most sophisticated instruments to study these relatively close systems it’s difficult to obtain complete data so typically one or two potential protoplanets get identified.

In the case of the HD 169142 system, the star’s estimated age is about 6 million years, little more than a newborn, and its mass is about 1.7 times the Sun’s. The use of the powerful radio telescope ALMA, inaugurated in march 2013, allowed to study the gas and dust disk surrounding the star and above all to understand how gas and dust are distributed. It’s not surprising that the distribution shows a ring structure typical of the planetary formation phase.

In particular, two rings, the first with a distance from its star between 20 and 35 times that of the Earth from the Sun and the second with a distance from its star between 56 and 83 times that of the Earth from the Sun, have been identified. There are also intermediate zones within the disk in which gas and dust have been cleaned and it’s there that astronomers go to look for the planets in formation.

Alternative hypothesis have been proposed to determine if there may be other phenomena that lead to the creation of those cleaned areas between the rings. The possibilities are for example magnetorotational instability or the fusing of dust grains but the presence of planets in formation is the most plausible. Other observations of the HD 169142 system were carried out at visible and infrared wavelengths with results compatible with the presence of two protoplanets.

The ALMA radio telescope allowed to also study the distribution of gas in the protoplanetary disk through the molecular analysis of three isotopes of carbon monoxide. By mapping the gas intensity and the differences compared to the dust intensity, the researchers concluded that there are two protoplanets, one distant from its star 15 times the distance of the Earth from the Sun and one that is 50 times the Earth from the Sun.

The estimates of the masses of the two protoplanets are very approximate: the internal one has a mass between one-tenth and one mass of the planet Jupiter and the external one has a mass between one and ten masses of the planet Jupiter. The HD 169142 system needs to be studied further in order to obtain more accurate estimates but there’s all the time because the planet-forming process will continue for millions of years.

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