added stao.ipynb and exoplanets.py for adding exoplanet systems

exoplanets.py

@@ -0,0 +1,35 @@
+import pandas as pd
+import rebound 
+import numpy as np
+
+def add_planet(sim, planet, T1):
+    P = planet['pl_orbper']/365.25
+    e = planet['pl_orbeccen']
+    pomega = planet['pl_orbeccen']/(2.*np.pi)
+    t_transit = (planet['pl_tranmid']-T1)/365.25 # time of transit relative to first planet, defined as t=0
+    for el in [P, e, pomega, t_transit]:
+        if np.isnan(el) is True:
+            el = 0.
+    sim.integrate(t_transit)
+    sim.add(P=P, e=e, pomega=pomega, theta=0) # add new planet at true longitude=0 (transiting)
+
+def make_sim(df, name):
+    system = df[df['pl_hostname'] == name]
+    if system.shape[0] == 0:
+        raise AttributeError("System {0} not found in NASA exoplanet archive".format(name))
+
+    p1 = system.iloc[0]
+    starmass = p1['st_mass']
+    sim = rebound.Simulation()
+    sim.G = 4*np.pi**2
+    sim.add(m=starmass)
+
+    T1 = p1['pl_tranmid'] # time of mid-transit for first planet
+    for i, planet in system.iterrows():
+        add_planet(sim, planet, T1)
+
+    Ps = [sim.particles[i].P for i in range(1, sim.N)]
+    Pmin = np.array(Ps).min()
+    sim.integrator="whfast"
+    sim.dt = Pmin*0.07
+    return sim