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El Sistema Planetario De Trappist-1

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El Sistema Planetario De Trappist-1 (#313). EL SISTEMA PLANETARIO DE TRAPPIST-1 [MONOTEMA] El descubrimiento del sistema TRAPPIST-1 ha generado gran interés debido, fundamentalmente, a que los siete planetas detectados tienen características similares a la Tierra y podrían albergar vida. Esos planetas orbitan alrededor de una estrella enana tipo M, y además lo hacen de manera resonante y con acoplamiento por fuerzas de marea. En este post describo los detalles más relevantes de este hallazgo, y explico de forma breve los principales métodos de detección empleados en este campo de la física, así como también analizo la posible habitabilidad de los planetas encontrados. Muestro, por tanto, de manera didáctica, la complejidad de obtener resultados concluyentes cuando todavía existe incertidumbre importante en los datos, pero al mismo tiempo ilustro que a través de la observación, simulación numérica y análisis estadísticos, se puede obtener un dibujo aproximado de una parte de la galaxia que está a unos 39 años luz de nosotros. Introducción El descubrimiento de tres exoplanetas en la zona habitable de la estrella TRAPPIST-1A por Gillon et al. (2016), ha sido uno de los eventos más destacados en el campo de la astrofísica en los últimos años. Tan sólo unos meses más tarde, Gillon et al. (2017) publicaron que habían detectado hasta 7 planetas orbitando esa estrella, los cuales podrían contener agua líquida. De este modo, el hallazgo de los exoplanetas de TRAPPIST-1 constituye un caso muy atractivo para ilustrar algunas de las actividades fundamentales de la investigación en astrofísica, así como para explicar conceptos clave asociados a la búsqueda de lugares en el espacio con el potencial de albergar vida. En este artículo voy a describir los detalles de ese descubrimiento, definiendo los términos más importantes que se emplean para su correcto entendimiento, y discutiendo algunas de las contradicciones que muestran los estudios. Lo haré además, de manera didáctica, con el fin de facilitar su conocimiento. El objetivo de esta investigación es, por tanto, hacer comprensible el significado de este hallazgo científico, analizando brevemente las múltiples implicaciones que de él se derivan. Descripción del sistema planetario de TRAPPIST-1 Gillon et al. (2016) monitorizaron el brillo de la estrella TRAPPIST-1A durante 245 horas en 62 noches desde el 17 de septiembre al 28 de diciembre de 2015. Esa estrella se encuentra a una distancia estimada de la Tierra de 39.46 años- luz, y es una enana ultra fría tipo M situada en la constelación de Acuario. Su edad se estima en 7.6 ± 2.2 Gyr (Burgasser & Mamajek, 2017), es decir, de varios miles de millones de años, y es ciertamente más pequeña y menos brillante que el Sol. Así, Gillon et al. (2016) describieron su luminosidad, masa y radio como 0.05%, 8% y 11.5% en relación al Sol, respectivamente (los datos actualizados sobre todos los parámetros se encuentran en www.trappist.one). Como indica Sholkmann (2017), este descubrimiento ha sido posible gracias al trabajo conjunto de varios telescopios terrestres y espaciales: TRAPPIST TRansiting( Planets and PlanestIsimals Small Telescope) North system (Chile), el telescopio TRAPPIST-North (Marruecos), el Himalayan Chandra Telescope (India), el Very Large Telescope (Chile), el UK Infrared Telescope (Hawaii), el Spitzer Space Telescope, los telescopios William Herschel y Liverpool (La Palma, España), y el telescopio del South African Astronomical Observatory. A través del método de tránsito primario, Gillon et al. (2016) detectaron inicialmente 3 exoplanetas orbitando la estrella: TRAPPIST-1b, TRAPPIST-1c y TRAPPIST-1d. Posteriormente, Gillon et al. (2017) reportaban la detección de 4 exoplanetas más: TRAPPIST-1e, TRAPPIST-1f, TRAPPIST-1g, y TRAPPIST-1h, completando un sistema de 7 planetas que, al menos en número, es similar al Sistema Solar. La cadencia temporal del proceso de descubrimiento y la publicación de los estudios en revistas científicas puede consultarse en www.trappist.one, así como las características actualizadas de esos planetas (en las Tablas 1 y 2 mostramos algunas de ellas). Tabla 1. Características de la estrella TRAPPIST-1A Ascensión α = 23h 06m 0.117 ± 0.004 Radio recta 29.28s R☉ δ = -05º 02′ 50.7 (-2.2, Declinación Densidad 28.5” +1.2) ρ☉ Temperatura Constelación Acuario 2559 ± 50 K efectiva 0.000525 ± Distancia 12.1 ± 0.4 pc Luminosidad 0.000036 L☉ 0.080 ± 0.007 Masa Edad 7.6 ± 2.2 Gyr M☉ 1 Gyr = 1000 millones de años; 1 pc (Parsec) = 3.26156 años/luz; El subíndice ☉ se refiere al equivalente del Sol de la medida empleada. Fuente: www.trappist.one/#system Tabla 2. Características de los planetas de la estrella TRAPPIST-1A 1b 1c 1d 1e 1f 1g 1h 1.51087081 2.4218233 9.206690 12.35294 Periodo ± ± 4.049610 ± 6.099615 ± ± ± 18.767 ± orbital 0.00000060 0.0000017 0.000063días 0.000011días 0.000015 0.00012 0.004 días días días días días 36.40 ± 42.37 ± 62.60 ± 68.40 ± Duración del 49.13 ± 0.65 57.21 ± 0.71 75.6 ± 1.2 0.17 0.22 0.60 0.66 tránsito minutos minutos minutos minutos minutos minutos minutos Inclinación 89.65 ± 89.67 ± 89.75 ± 89.86 ± 89.680 ± 89.710 ± 89.76 ± orbital 0.25º 0.17º 0.16º 0.11º 0.034º 0.025º 0.05º Excentricidad < 0.081 < 0.083 < 0.070 < 0.085 < 0.063 < 0.061 Desconocida orbital 0.01521 ± 0.0451 ± 0.01111 ± 0.02144 ± 0.02817 ± 0371 ± 0.059 ± Semieje mayor 0.00047 0.0014 0.00034 UA 0.00065 UA 0.00085 UA 0.001 0.003 UA UA UA 1.086 ± 1.056 ± 0.772 ± 0.918 ± 1.045 ± 1.127 ± 0.752 ± Radio 0.035 R⊕ 0.035 R⊕ 0.030 R⊕ 0.039 R⊕ 0.038 R⊕ 0.041 R⊕ 0.032 R⊕ 0.85 ± 1.38 ± 0.41 ± 0.27 0.62 ± 0.58 0.68 ± 1.34 ± Masa Desconocida 0.72 M⊕ 0.61 M⊕ M⊕ M⊕ 0.18 M⊕ 0.88 M⊕ 0.66 ± 1.17 ± 0.89 ± 0.60 0.80 ± 0.76 0.60 ± 0.94 ± Densidad Desconocida 0.56 ρ⊕ 0.53 ρ⊕ ρ⊕ ρ⊕ 0.17 ρ⊕ 0.63 ρ⊕ 4.25 ± 2.27 ± 1.143 ± 0.662 ± 0.382 ± 0.258 ± 0.165 ± Irradiación 0.33 S⊕ 0.18 S⊕ 0.088 S⊕ 0.051 S⊕ 0.030 S⊕ 0.020 S⊕ 0.025 Temperatura 400.1 ± 341.9 ± 288.0 ± 5.6 251.3 ± 4.9 219.0 ± 198.6 ± 173 ± 4 K de equilibrio 7.7 K 6.6 K K K 4.2 K 3.8 K 1 UA (Unidad Astronómica) = 149 597 870 700 m (equivale a la distancia media entre la Tierra y el Sol); El subíndice ⊕ se refiere al equivalente de la Tierra de la medida empleada. Fuente: www.trappist.one/#system Un análisis sencillo de la Tabla 2 nos indica que los planetas de TRAPPIST-1 tienen una inclinación prácticamente perpendicular al cielo (visto desde la Tierra). Los periodos orbitales son de muy pocos días, dada la corta distancia con respecto a la estrella que podemos tomar de la medida del semieje mayor. Periodo y semieje mayor se relacionan por la Tercera Ley de Kepler. Todos los planetas tienen órbitas más excéntricas que la Tierra; en nuestro planeta la excentricidad es de 0.0167, es decir, casi circular. No obstante, tal y como comentan Gupta et al. (2001), la Tierra ha sufrido ciclos de excentricidad, por lo que no es un parámetro constante. Radios, masas y densidades son similares a las de nuestro planeta, aunque los datos para TRAPPIST-1h son mucho más inciertos. En cuanto a la irradiación y a la temperatura de equilibrio, existe gran variación debido a la diferencia en distancias con respecto a la estrella. Recordemos que la temperatura de equilibrio de la Tierra considerando el albedo (porcentaje de radiación reflejada) es de 254 K (vanLoon & Duffy, 2011). Por tanto, los planetas TRAPPIST-1d, TRAPPIST-e y TRAPPIST-1f, tendrían temperaturas de equilibrio ciertamente similares a la Tierra. La irradiación juega un papel fundamental en la consideración de potencial habitabilidad del planeta, como posteriormente veremos. La correlación entre varios de los parámetros de los exoplanetas es altamente compleja. Por ejemplo, Mancini (2017) encontró una posible asociación entre el radio y la temperatura de equilibrio, aunque esa relación no se muestra para el caso de TRAPPIST-1, ya que la correlación de Spearman (ρ) es no significativa ρ = 0.32 (p=0.48), Méndez & Rivera- Valentín (2017), por su parte, indicaron que existe una correlación positiva entre la excentricidad de la órbita y la irradiación recibida de la estrella, y encuentran que la temperatura de equilibrio media de un exoplaneta decrece con la excentricidad. Para el caso de TRAPPIST-1,ρ = 0.6 (p=0.21), para ambas correlaciones, por lo que tampoco son estadísticamente significativas. En cuanto a la relación entre la densidad y el radio, Weiss & Marcy (2014) encontraron que para planetas con radios menores a 1.5 R⊕, como el caso de los planetas detectados en TRAPPIST-1, la densidad se incrementa con el radio de manera lineal. Sin embargo, ρ = 0.2 (p=0.70) y la correlación lineal de Pearson r=-0.23 (p=0.94), es decir, resultados de nuevo no significativos. No obstante, lo ideal sería volver a calcular esas correlaciones con los datos brutos (pero ya recalibrados) sobre cada medición, y no sobre los parámetros medios presentes en la Tabla 2. Así, se tendrían muchas más observaciones y unos errores estándares mucho más pequeños, y de este modo mayor potencia para detectar asociaciones significativas. TRAPPIST-1 presenta resonancia orbital entre sus planetas. Este es un fenómeno que indica que existe una relación fraccional de números enteros entre los periodos de revolución de las órbitas.
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