How to add an extra column to a NumPy array

Let’s say I have a NumPy array, a:

a = np.array([
    [1, 2, 3],
    [2, 3, 4]
    ])

And I would like to add a column of zeros to get an array, b:

b = np.array([
    [1, 2, 3, 0],
    [2, 3, 4, 0]
    ])

How can I do this easily in NumPy?