Metodo fotometrico di transito
Il metodo fotometrico di transito, consiste nella rilevazione della diminuzione di luminosità della curva di luce di una stella, quando un pianeta transita di fronte alla stella madre. La diminuzione è correlata alla dimensione relativa della stella madre, del pianeta e della sua orbita.
Anche se questo metodo funziona solo per i pianeti la cui orbita è perfettamente allineata col nostro punto di vista, può essere utilizzato fino a grandi distanze.
Ecco per esempio un’animazione che mostra un andamento tipico della variazione di luminosità (curva di luce) per un sistema con un solo esopianeta:
Fonte: http://astroutils.astronomy.ohio-state.edu/exofast
Nella parte superiore è rappresentata l’orbita del pianeta lungo la direzione di vista dell’osservatore, mentre nella parte inferiore viene disegnata la curva di luce rilevata da un sensore nel tempo. Dal grafico si distinguono le seguenti fasi:
- Primo e secondo punto di contatto e fase di immersione: calo progressivo del flusso di luminosità della stella.
- Luminosità più bassa per tutta la durata del transito.
- Terzo e quarto punto di contatto e fase di emersione: la luminosità della stella ospite torna ai valori iniziali.
Il primo pianeta ad essere stato scoperto grazie al metodo dei transiti è stato HD209458 b.
Un altro esempio di esopianeti scoperti grazie a questo metodo sono quelli di Trappist-1:
Grazie l’osservazione al telescopio della stella per un certo periodo e dalla registrazione della quantità di luce proveniente dalla stella, si sono visti dei cali periodici nella curva luminosa, dovuti ai pianeti che gli orbitano attorno. Misurando quanto tempo impiegano i pianeti a passare davanti alla stella, quanta luce bloccano e con quale frequenza orbitano, è stato possibile calcolare le masse di questi pianeti e quanto sono lontani dalla stella, usando le leggi del moto di Keplero. Inizialmente, dai dati erano stati determinati almeno 3 pianeti, uno di questi pianeti nella zona abitabile della stella. In seguito dopo alcune misurazioni più precise della NASA, con un telescopio a infrarossi (Spitzer), è stata stabilita la presenza di almeno 7 pianeti in orbita attorno a Trappist-1, 3 dei quali nella zona abitabile.
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