In the plane a point $O$ is and a sequence of points $P_1, P_2, P_3, \ldots$ are given. The distances $OP_1, OP_2, OP_3, \ldots$ are $r_1, r_2, r_3, \ldots$ Let $\alpha$ satisfies $0 < \alpha < 1.$ Suppose that for every $n$ the distance from the point $P_n$ to any other point of the sequence is $\geq r^{\alpha}_n.$ Determine the exponent $\beta$, as large as possible such that for some $C$ independent of $n$ 
\[r_n \geq Cn^{\beta}, n = 1,2, \ldots\]
1. **Understanding the Problem:**
   We are given a point \( O \) and a sequence of points \( P_1, P_2, P_3, \ldots \) in the plane. The distances from \( O \) to \( P_n \) are denoted by \( r_n \). We are also given that for every \( n \), the distance from \( P_n \) to any other point in the sequence is at least \( r_n^\alpha \), where \( 0 < \alpha < 1 \). We need to determine the largest possible exponent \( \beta \) such that \( r_n \geq Cn^\beta \) for some constant \( C \) independent of \( n \).

2. **Assumption and Condition:**
   We assume the additional condition \( r_1 \leq r_2 \leq r_3 \leq \cdots \) as mentioned in the IMO Compendium. This condition ensures that the sequence \( r_n \) is non-decreasing.

3. **Analyzing the Distance Condition:**
   The condition that the distance from \( P_n \) to any other point in the sequence is at least \( r_n^\alpha \) implies that the points \( P_n \) are spread out in the plane. Specifically, for any \( m \neq n \), the distance \( P_nP_m \geq r_n^\alpha \).

4. **Bounding \( r_n \):**
   To find the largest possible \( \beta \), we need to analyze how \( r_n \) grows with \( n \). We use the fact that the points \( P_n \) are spread out and the distances \( r_n \) are non-decreasing.

5. **Using the Distance Condition:**
   Consider the circle of radius \( r_n^\alpha \) centered at \( P_n \). No other point \( P_m \) (for \( m \neq n \)) can lie within this circle. This implies that the area around each point \( P_n \) is essentially "forbidden" for other points.

6. **Estimating the Growth of \( r_n \):**
   The area of the circle with radius \( r_n^\alpha \) is \( \pi (r_n^\alpha)^2 = \pi r_n^{2\alpha} \). Since the points are spread out, the total area covered by \( n \) such circles should be proportional to the area of a larger circle with radius \( r_n \). Thus, we have:
   \[
   n \cdot \pi r_n^{2\alpha} \leq \pi r_n^2
   \]
   Simplifying, we get:
   \[
   n \leq r_n^{2 - 2\alpha}
   \]
   Taking the logarithm on both sides, we get:
   \[
   \log n \leq (2 - 2\alpha) \log r_n
   \]
   Solving for \( r_n \), we get:
   \[
   r_n \geq n^{\frac{1}{2 - 2\alpha}}
   \]

7. **Determining \( \beta \):**
   Comparing this with \( r_n \geq Cn^\beta \), we see that:
   \[
   \beta = \frac{1}{2 - 2\alpha}
   \]

8. **Conclusion:**
   The largest possible exponent \( \beta \) such that \( r_n \geq Cn^\beta \) for some constant \( C \) independent of \( n \) is:
   \[
   \beta = \frac{1}{2(1 - \alpha)}
   \]

The final answer is \( \boxed{ \beta = \frac{1}{2(1 - \alpha)} } \)